diff --git a/bintree.c b/bintree.c
index 5cf82a9..ecf5a5f 100644
--- a/bintree.c
+++ b/bintree.c
@@ -2,35 +2,183 @@
 #include "stack.h"
 #include "bintree.h"
 
-//TODO: binären Suchbaum implementieren
-/*  * `addToTree`: fügt ein neues Element in den Baum ein (rekursiv),
-    * `clearTree`: gibt den gesamten Baum frei (rekursiv),
-    * `treeSize`: zählt die Knoten im Baum (rekursiv),
-    * `nextTreeData`: Traversierung mit Hilfe des zuvor implementierten Stacks. */
+// Global static stack used for in-order tree traversal in nextTreeData()
+// This is needed because nextTreeData() follows strtok-like usage where consecutive calls
+// with NULL argument continue the iteration
+static StackNode *traversalStack = NULL;
 
-// Adds a copy of data's pointer destination to the tree using compareFct for ordering. Accepts duplicates
-// if isDuplicate is NULL, otherwise ignores duplicates and sets isDuplicate to 1 (or to 0 if a new entry is added).
+// Adds a copy of data's pointer destination to the tree using compareFct for ordering.
+// Accepts duplicates if isDuplicate is NULL, otherwise ignores duplicates and sets isDuplicate to 1 (or to 0 if a new entry is added).
+// Parameters:
+//   - root: pointer to the root of the tree (can be NULL for first insertion)
+//   - data: pointer to the data to be inserted
+//   - dataSize: size in bytes of the data to allocate and copy
+//   - compareFct: comparison function that returns <0 if arg1<arg2, 0 if equal, >0 if arg1>arg2
+//   - isDuplicate: pointer to int flag (NULL to allow duplicates, non-NULL to detect and reject duplicates)
+// Returns: pointer to the root of the modified tree
 TreeNode *addToTree(TreeNode *root, const void *data, size_t dataSize, CompareFctType compareFct, int *isDuplicate)
 {
-
+    // Base case: if root is NULL, we've found the insertion point
+    // Create a new tree node here
+    if (root == NULL)
+    {
+        // Allocate memory for the new tree node
+        TreeNode *newNode = (TreeNode *)malloc(sizeof(TreeNode));
+        if (newNode == NULL)
+        {
+            return NULL;  // Allocation failed
+        }
+        
+        // Allocate memory for a copy of the data
+        newNode->data = malloc(dataSize);
+        if (newNode->data == NULL)
+        {
+            free(newNode);  // Free the node if data allocation failed
+            return NULL;
+        }
+        
+        // Copy the data into the newly allocated space
+        memcpy(newNode->data, data, dataSize);
+        
+        // Initialize the left and right child pointers to NULL
+        newNode->left = NULL;
+        newNode->right = NULL;
+        
+        // If isDuplicate is not NULL, set it to 0 to indicate a new entry was added
+        if (isDuplicate != NULL)
+        {
+            *isDuplicate = 0;
+        }
+        
+        // Return the newly created node
+        return newNode;
+    }
+    
+    // Recursive case: compare the data with the root's data to determine where to insert
+    int comparisonResult = compareFct(data, root->data);
+    
+    // If data is less than root->data, insert into the left subtree
+    if (comparisonResult < 0)
+    {
+        root->left = addToTree(root->left, data, dataSize, compareFct, isDuplicate);
+    }
+    // If data is greater than root->data, insert into the right subtree
+    else if (comparisonResult > 0)
+    {
+        root->right = addToTree(root->right, data, dataSize, compareFct, isDuplicate);
+    }
+    // If data equals root->data, we have a duplicate
+    else
+    {
+        // If isDuplicate is not NULL, set it to 1 to indicate a duplicate was found
+        if (isDuplicate != NULL)
+        {
+            *isDuplicate = 1;
+        }
+        // If isDuplicate is NULL, we allow the duplicate to be inserted again (in either subtree)
+        // For standard BST behavior, we typically insert duplicates in the right subtree
+        else
+        {
+            root->right = addToTree(root->right, data, dataSize, compareFct, isDuplicate);
+        }
+    }
+    
+    // Return the root (possibly with new subtrees added)
+    return root;
 }
 
-// Iterates over the tree given by root. Follows the usage of strtok. If tree is NULL, the next entry of the last tree given is returned in ordering direction.
-// Use your implementation of a stack to organize the iterator. Push the root node and all left nodes first. On returning the next element,
-// push the top node and push all its left nodes.
+// Iterates over the tree given by root in in-order traversal (Left, Root, Right).
+// Follows the usage pattern of strtok: first call with root pointer, subsequent calls with NULL to get next element.
+// Uses a static stack to maintain traversal state between calls.
+// Parameters:
+//   - root: pointer to tree root for first call, NULL for subsequent calls
+// Returns: pointer to the data of the next node in in-order sequence, or NULL when traversal is complete
 void *nextTreeData(TreeNode *root)
 {
-
+    // If root is not NULL, we're starting a new traversal
+    if (root != NULL)
+    {
+        // Clear any existing traversal state
+        clearStack(traversalStack);
+        traversalStack = NULL;
+        
+        // Initialize the traversal by pushing root and all its left descendants
+        // In in-order traversal, we always visit left children first
+        TreeNode *current = root;
+        while (current != NULL)
+        {
+            // Push current node onto the stack
+            traversalStack = push(traversalStack, current);
+            // Move to the left child
+            current = current->left;
+        }
+    }
+    
+    // If the traversal stack is empty, we've visited all nodes
+    if (traversalStack == NULL)
+    {
+        return NULL;
+    }
+    
+    // Pop the next node from the stack
+    TreeNode *nextNode = (TreeNode *)top(traversalStack);
+    traversalStack = pop(traversalStack);
+    
+    // If the popped node has a right child, push it and all its left descendants
+    // This handles the "Root, Right" part of in-order traversal
+    if (nextNode->right != NULL)
+    {
+        TreeNode *current = nextNode->right;
+        while (current != NULL)
+        {
+            // Push the current node
+            traversalStack = push(traversalStack, current);
+            // Move to the left child of the right subtree
+            current = current->left;
+        }
+    }
+    
+    // Return the data of the node we just processed
+    return nextNode->data;
 }
 
-// Releases all memory resources (including data copies).
+// Releases all memory resources used by the tree.
+// Recursively frees all nodes and their data copies.
+// Parameters:
+//   - root: pointer to the root of the tree to clear
+// Returns: void
 void clearTree(TreeNode *root)
 {
-
+    // Base case: if root is NULL, there's nothing to free
+    if (root == NULL)
+    {
+        return;
+    }
+    
+    // Recursive case: first recursively clear the left subtree
+    clearTree(root->left);
+    
+    // Then recursively clear the right subtree
+    clearTree(root->right);
+    
+    // Finally, free the data and the node itself
+    free(root->data);
+    free(root);
 }
 
-// Returns the number of entries in the tree given by root.
+// Returns the number of entries (nodes) in the tree.
+// Recursively counts nodes in the left and right subtrees.
+// Parameters:
+//   - root: pointer to the root of the tree
+// Returns: unsigned int representing the total number of nodes in the tree
 unsigned int treeSize(const TreeNode *root)
 {
-
+    // Base case: if root is NULL, there are no nodes
+    if (root == NULL)
+    {
+        return 0;
+    }
+    
+    // Recursive case: count 1 for the current node, plus all nodes in left and right subtrees
+    return 1 + treeSize(root->left) + treeSize(root->right);
 }
\ No newline at end of file
diff --git a/bintree.o b/bintree.o
new file mode 100644
index 0000000..a1a30ce
Binary files /dev/null and b/bintree.o differ
diff --git a/bintree_tests b/bintree_tests
new file mode 100755
index 0000000..a8e151a
Binary files /dev/null and b/bintree_tests differ
diff --git a/doble_initial b/doble_initial
new file mode 100755
index 0000000..d60f79b
Binary files /dev/null and b/doble_initial differ
diff --git a/highscores.txt b/highscores.txt
index 4edd5a7..a6a0d3e 100644
--- a/highscores.txt
+++ b/highscores.txt
@@ -1 +1,2 @@
+player_name;6999
 player1;3999
diff --git a/makefile b/makefile
index 97c5e30..7a7d9d2 100644
--- a/makefile
+++ b/makefile
@@ -38,12 +38,27 @@ $(program_obj_files): %.o: %.c
 unitTests:
 	echo "needs to be implemented"
 
+# --------------------------
+# Test: test_numbers
+# --------------------------
+test_numbers: stack.o bintree.o numbers.o unity.o
+	$(CC) $(FLAGS) -Iunity test_numbers.c $^ -o runTest_numbers
+
+unity.o:
+	$(CC) -c $(FLAGS) -Iunity unity/unity.c -o unity.o
+
+# --------------------------
+# Test: test_bintree
+# --------------------------
+test_bintree: stack.o bintree.o unity.o
+	$(CC) $(FLAGS) -Iunity test_bintree.c $^ -o runTest_bintree
+
 # --------------------------
 # Clean
 # --------------------------
 clean:
 ifeq ($(OS),Windows_NT)
-	del /f *.o doble
+	del /f *.o doble runTest_numbers runTest_bintree
 else
-	rm -f *.o doble
+	rm -f *.o doble runTest_numbers runTest_bintree
 endif
\ No newline at end of file
diff --git a/numbers.c b/numbers.c
index f59d9a2..6fcd80c 100644
--- a/numbers.c
+++ b/numbers.c
@@ -11,16 +11,151 @@
   * Duplizieren eines zufälligen Eintrags im Array.
   * in `getDuplicate()`: Sortieren des Arrays und Erkennen der doppelten Zahl durch Vergleich benachbarter Elemente. */
 
+// Comparison function for unsigned integers used in the binary search tree
+// Returns: <0 if a < b, 0 if a == b, >0 if a > b
+// This function is passed to addToTree to determine ordering
+int compareUnsignedInts(const void *a, const void *b)
+{
+    unsigned int valA = *(const unsigned int *)a;
+    unsigned int valB = *(const unsigned int *)b;
+    
+    // Simple subtraction works for unsigned integers in this case
+    // (Note: in general subtraction can overflow, but our range is small)
+    if (valA < valB) return -1;
+    if (valA > valB) return 1;
+    return 0;
+}
+
 // Returns len random numbers between 1 and 2x len in random order which are all different, except for two entries.
 // Returns NULL on errors. Use your implementation of the binary search tree to check for possible duplicates while
 // creating random numbers.
+//
+// Algorithm:
+// 1. Generate len-1 unique random numbers using the BST for duplicate detection
+// 2. Randomly duplicate one of those numbers by inserting it at a random position
+// 3. Final array has len entries with exactly one value appearing twice
 unsigned int *createNumbers(unsigned int len)
 {
+    // Step 1: Validate input - we need at least 2 elements to create a duplicate
+    if (len < 2)
+    {
+        return NULL;
+    }
+    
+    // Step 2: Allocate memory for the final array (len elements total)
+    unsigned int *numbers = (unsigned int *)malloc(len * sizeof(unsigned int));
+    if (numbers == NULL)
+    {
+        return NULL;
+    }
+    
+    // Step 3: Create binary search tree to track generated numbers and prevent accidental duplicates
+    // Note: Random seed should be initialized once in main() with srand(time(NULL))
+    // Calling srand() here would reset the seed each time, reducing randomness
+    TreeNode *usedNumbers = NULL;
+    
+    // Step 4: Generate len-1 unique random numbers (we'll add one duplicate later)
+    unsigned int arrayIndex = 0;
+    
+    while (arrayIndex < len - 1)
+    {
+        // Generate random number in range [1, 2×len]
+        unsigned int randomNumber = (rand() % (2 * len)) + 1;
+        
+        // Check if this number was already generated
+        int isDuplicate = 0;
+        usedNumbers = addToTree(usedNumbers, &randomNumber, sizeof(unsigned int), compareUnsignedInts, &isDuplicate);
+        
+        if (usedNumbers == NULL)
+        {
+            free(numbers);
+            return NULL;
+        }
+        
+        // If new number, add to result array; if duplicate, retry
+        if (isDuplicate == 0)
+        {
+            numbers[arrayIndex] = randomNumber;
+            arrayIndex++;
+        }
+    }
+    
+    // Step 5: Pick a random value from our generated numbers to duplicate
+    unsigned int indexToDuplicate = rand() % (len - 1);
+    unsigned int valueToDuplicate = numbers[indexToDuplicate];
+    
+    // Step 6: Pick a random position to insert the duplicate (anywhere in the array)
+    unsigned int insertPos = rand() % len;
+    
+    // Step 7: Shift elements right to make room for the duplicate
+    for (unsigned int i = len - 1; i > insertPos; i--)
+    {
+        numbers[i] = numbers[i - 1];
+    }
+    
+    // Step 8: Insert the duplicate at the chosen position
+    numbers[insertPos] = valueToDuplicate;
+    
+    // Step 9: Clean up the binary search tree (we don't need it anymore)
+    clearTree(usedNumbers);
+    
+    // Return the array with len elements (len-1 unique + 1 duplicate)
+    return numbers;
+}
 
+// Comparison function for qsort to sort unsigned integers in ascending order
+// Returns: <0 if a < b, 0 if a == b, >0 if a > b
+static int compareUnsigned(const void *a, const void *b)
+{
+    unsigned int valA = *(const unsigned int *)a;
+    unsigned int valB = *(const unsigned int *)b;
+    
+    if (valA < valB) return -1;
+    if (valA > valB) return 1;
+    return 0;
 }
 
 // Returns only the only number in numbers which is present twice. Returns zero on errors.
-unsigned int getDuplicate(const unsigned int numbers[], unsigned int len)
+//
+// Algorithm:
+// 1. Create a copy of the array to avoid modifying the input
+// 2. Sort the copy using qsort()
+// 3. Compare adjacent elements to find the duplicate
+// 4. Return the duplicate value, or 0 on error
+unsigned int getDuplicate(const unsigned int *numbers, unsigned int len)
 {
-
+    // Step 1: Input validation - need at least 2 elements to have a duplicate
+    if (numbers == NULL || len < 2)
+    {
+        return 0;
+    }
+    
+    // Step 2: Create a copy of the array to sort (we don't modify the input)
+    unsigned int *sorted = (unsigned int *)malloc(len * sizeof(unsigned int));
+    if (sorted == NULL)
+    {
+        return 0;
+    }
+    
+    // Step 3: Copy the array
+    memcpy(sorted, numbers, len * sizeof(unsigned int));
+    
+    // Step 4: Sort the copy
+    qsort(sorted, len, sizeof(unsigned int), compareUnsigned);
+    
+    // Step 5: Compare adjacent elements to find the duplicate
+    for (unsigned int i = 0; i < len - 1; i++)
+    {
+        if (sorted[i] == sorted[i + 1])
+        {
+            // Found the duplicate!
+            unsigned int result = sorted[i];
+            free(sorted);
+            return result;
+        }
+    }
+    
+    // Step 6: No duplicate found (should not happen if input is valid)
+    free(sorted);
+    return 0;
 }
\ No newline at end of file
diff --git a/numbers.o b/numbers.o
new file mode 100644
index 0000000..b6079d0
Binary files /dev/null and b/numbers.o differ
diff --git a/runTest_bintree b/runTest_bintree
new file mode 100755
index 0000000..32091ce
Binary files /dev/null and b/runTest_bintree differ
diff --git a/runTest_numbers b/runTest_numbers
new file mode 100755
index 0000000..545ec6c
Binary files /dev/null and b/runTest_numbers differ
diff --git a/stack.c b/stack.c
index e3a90d4..be93edb 100644
--- a/stack.c
+++ b/stack.c
@@ -1,33 +1,99 @@
 #include <stdlib.h>
 #include "stack.h"
 
-//TODO: grundlegende Stackfunktionen implementieren:
-/*  * `push`: legt ein Element oben auf den Stack,
-    * `pop`: entfernt das oberste Element,
-    * `top`: liefert das oberste Element zurück,
-    * `clearStack`: gibt den gesamten Speicher frei. */
-
 // Pushes data as pointer onto the stack.
+// Parameters:
+//   - stack: pointer to the current top of the stack (can be NULL if stack is empty)
+//   - data: void pointer to the data to be pushed onto the stack
+// Returns: pointer to the new top of the stack (the newly created node)
+// Note: The function allocates memory for a new StackNode, sets its data pointer
+//       and links it to the previous top, making it the new top of the stack.
 StackNode *push(StackNode *stack, void *data)
 {
-
+    // Allocate memory for a new StackNode
+    StackNode *newNode = (StackNode *)malloc(sizeof(StackNode));
+    
+    // Check if memory allocation was successful
+    if (newNode == NULL)
+    {
+        return NULL;  // Return NULL if allocation failed
+    }
+    
+    // Store the data pointer in the new node
+    newNode->data = data;
+    
+    // Link the new node to the previous top of the stack
+    // If stack was NULL (empty), newNode->next will be NULL
+    // If stack was not NULL, newNode->next points to the old top
+    newNode->next = stack;
+    
+    // Return the new node, which is now the new top of the stack
+    return newNode;
 }
 
-// Deletes the top element of the stack (latest added element) and releases its memory. (Pointer to data has to be
-// freed by caller.)
+// Deletes the top element of the stack (latest added element) and releases its memory.
+// Parameters:
+//   - stack: pointer to the current top of the stack
+// Returns: pointer to the new top of the stack (the second element, or NULL if stack becomes empty)
+// Note: This function only frees the StackNode structure itself. The caller is responsible
+//       for freeing the data that the node was pointing to, if that data was dynamically allocated.
 StackNode *pop(StackNode *stack)
 {
-
+    // Check if the stack is empty
+    if (stack == NULL)
+    {
+        return NULL;  // Cannot pop from an empty stack
+    }
+    
+    // Save a pointer to the next node (which will become the new top)
+    StackNode *newTop = stack->next;
+    
+    // Free the memory of the current top node
+    // NOTE: We do NOT free stack->data here because the caller may still need it
+    //       or may be responsible for freeing it themselves
+    free(stack);
+    
+    // Return the new top of the stack (or NULL if the stack is now empty)
+    return newTop;
 }
 
-// Returns the data of the top element.
+// Returns the data of the top element without removing it from the stack.
+// Parameters:
+//   - stack: pointer to the current top of the stack
+// Returns: void pointer to the data of the top element, or NULL if stack is empty
+// Note: This is a "peek" operation - it looks at the top without removing it.
 void *top(StackNode *stack)
 {
-
+    // Check if the stack is empty
+    if (stack == NULL)
+    {
+        return NULL;  // Return NULL if stack is empty
+    }
+    
+    // Return the data pointer stored in the top node
+    // The caller can then use this pointer to access the actual data
+    return stack->data;
 }
 
-// Clears stack and releases all memory.
+// Clears the entire stack and releases all memory used by the StackNodes.
+// Parameters:
+//   - stack: pointer to the current top of the stack
+// Returns: void
+// Note: This function only frees the StackNode structures. The caller is responsible
+//       for freeing any dynamically allocated data that the nodes were pointing to,
+//       unless that is done elsewhere in the program before calling clearStack.
 void clearStack(StackNode *stack)
 {
-
+    // Base case: if the stack is NULL (empty), we're done
+    if (stack == NULL)
+    {
+        return;
+    }
+    
+    // Recursive case: first, recursively clear all the nodes below this one
+    // This processes the stack from top to bottom
+    clearStack(stack->next);
+    
+    // After recursively clearing all deeper nodes, free the current node
+    free(stack);
 }
\ No newline at end of file
diff --git a/stack.h b/stack.h
index f7d542d..77993da 100644
--- a/stack.h
+++ b/stack.h
@@ -7,7 +7,12 @@ The latest element is taken from the stack. */
 
 #include <stdlib.h>
 
-//TODO: passenden Datentyp als struct anlegen
+// Stack node structure: each node contains data and a pointer to the next node
+typedef struct stackNode
+{
+    void *data;
+    struct stackNode *next;
+} StackNode;
 
 // Pushes data as pointer onto the stack.
 StackNode *push(StackNode *stack, void *data);
diff --git a/stack.o b/stack.o
new file mode 100644
index 0000000..5676a8c
Binary files /dev/null and b/stack.o differ
diff --git a/test_bintree.c b/test_bintree.c
new file mode 100644
index 0000000..a889753
--- /dev/null
+++ b/test_bintree.c
@@ -0,0 +1,592 @@
+#include "unity.h"
+#include "bintree.h"
+#include <stdlib.h>
+#include <string.h>
+
+/* ============================================================================
+ * UNIT TESTS FOR BINARY SEARCH TREE IMPLEMENTATION (Binärer Suchbaum)
+ * 
+ * This test suite covers all four functions:
+ * - addToTree(): Insert elements maintaining BST ordering
+ * - clearTree(): Free all memory in the tree
+ * - treeSize(): Count nodes in the tree
+ * - nextTreeData(): In-order traversal using stack
+ * ========================================================================== */
+
+// ============================================================================
+// HELPER FUNCTIONS FOR TESTING
+// ============================================================================
+
+// Comparison function for integers (ascending order: smaller values first)
+// Returns: <0 if a < b, 0 if a == b, >0 if a > b
+int compareIntegers(const void *a, const void *b)
+{
+    int valA = *(const int *)a;
+    int valB = *(const int *)b;
+    return valA - valB;
+}
+
+// Comparison function for strings (alphabetical order)
+int compareStrings(const void *a, const void *b)
+{
+    return strcmp((const char *)a, (const char *)b);
+}
+
+// ============================================================================
+// TEST SETUP AND TEARDOWN
+// ============================================================================
+
+void setUp(void)
+{
+    // Perform any setup needed before each test
+}
+
+void tearDown(void)
+{
+    // Perform any cleanup needed after each test
+    // Individual tests are responsible for clearing their trees
+}
+
+// ============================================================================
+// TEST SUITE 1: addToTree() - Inserting Elements
+// ============================================================================
+
+// Test 1.1: Insert a single element into an empty tree (NULL root)
+void test_addToTree_SingleElement(void)
+{
+    TreeNode *root = NULL;
+    int value = 42;
+    int isDuplicate = 0;
+    
+    // Insert the first element into an empty tree
+    root = addToTree(root, &value, sizeof(int), compareIntegers, &isDuplicate);
+    
+    // Verify the root is not NULL
+    TEST_ASSERT_NOT_NULL(root);
+    
+    // Verify the data was copied correctly
+    TEST_ASSERT_EQUAL_INT(42, *(int *)root->data);
+    
+    // Verify isDuplicate was set to 0 (new entry added)
+    TEST_ASSERT_EQUAL_INT(0, isDuplicate);
+    
+    // Verify the node has no children
+    TEST_ASSERT_NULL(root->left);
+    TEST_ASSERT_NULL(root->right);
+    
+    // Cleanup
+    clearTree(root);
+}
+
+// Test 1.2: Insert multiple elements in ascending order
+void test_addToTree_AscendingOrder(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {10, 20, 30};
+    
+    // Insert values in ascending order
+    for (int i = 0; i < 3; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+    }
+    
+    // Verify root value
+    TEST_ASSERT_EQUAL_INT(10, *(int *)root->data);
+    
+    // Verify each value goes to the right (since they're ascending)
+    TEST_ASSERT_NULL(root->left);
+    TEST_ASSERT_NOT_NULL(root->right);
+    TEST_ASSERT_EQUAL_INT(20, *(int *)root->right->data);
+    
+    TEST_ASSERT_NULL(root->right->left);
+    TEST_ASSERT_NOT_NULL(root->right->right);
+    TEST_ASSERT_EQUAL_INT(30, *(int *)root->right->right->data);
+    
+    // Cleanup
+    clearTree(root);
+}
+
+// Test 1.3: Insert multiple elements in descending order
+void test_addToTree_DescendingOrder(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {30, 20, 10};
+    
+    // Insert values in descending order
+    for (int i = 0; i < 3; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+    }
+    
+    // Verify root value
+    TEST_ASSERT_EQUAL_INT(30, *(int *)root->data);
+    
+    // Verify each value goes to the left (since they're descending)
+    TEST_ASSERT_NULL(root->right);
+    TEST_ASSERT_NOT_NULL(root->left);
+    TEST_ASSERT_EQUAL_INT(20, *(int *)root->left->data);
+    
+    TEST_ASSERT_NULL(root->left->right);
+    TEST_ASSERT_NOT_NULL(root->left->left);
+    TEST_ASSERT_EQUAL_INT(10, *(int *)root->left->left->data);
+    
+    // Cleanup
+    clearTree(root);
+}
+
+// Test 1.4: Insert elements in balanced order (creating a balanced tree)
+void test_addToTree_BalancedTree(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {20, 10, 30, 5, 15, 25, 35};
+    
+    // Insert values to create a somewhat balanced tree
+    for (int i = 0; i < 7; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+    }
+    
+    // Verify root value
+    TEST_ASSERT_EQUAL_INT(20, *(int *)root->data);
+    
+    // Verify left subtree
+    TEST_ASSERT_EQUAL_INT(10, *(int *)root->left->data);
+    TEST_ASSERT_EQUAL_INT(5, *(int *)root->left->left->data);
+    TEST_ASSERT_EQUAL_INT(15, *(int *)root->left->right->data);
+    
+    // Verify right subtree
+    TEST_ASSERT_EQUAL_INT(30, *(int *)root->right->data);
+    TEST_ASSERT_EQUAL_INT(25, *(int *)root->right->left->data);
+    TEST_ASSERT_EQUAL_INT(35, *(int *)root->right->right->data);
+    
+    // Cleanup
+    clearTree(root);
+}
+
+// Test 1.5: Attempt to insert a duplicate when isDuplicate is NULL (allows duplicates)
+void test_addToTree_AllowDuplicates(void)
+{
+    TreeNode *root = NULL;
+    int value1 = 42;
+    int value2 = 42;
+    
+    // Insert first instance
+    root = addToTree(root, &value1, sizeof(int), compareIntegers, NULL);
+    
+    // Insert duplicate (isDuplicate is NULL, so duplicates are allowed)
+    root = addToTree(root, &value2, sizeof(int), compareIntegers, NULL);
+    
+    // Verify tree has both entries (duplicate goes right)
+    TEST_ASSERT_EQUAL_INT(42, *(int *)root->data);
+    TEST_ASSERT_NULL(root->left);
+    TEST_ASSERT_NOT_NULL(root->right);
+    TEST_ASSERT_EQUAL_INT(42, *(int *)root->right->data);
+    
+    // Cleanup
+    clearTree(root);
+}
+
+// Test 1.6: Attempt to insert a duplicate when isDuplicate is provided (rejects duplicates)
+void test_addToTree_RejectDuplicates(void)
+{
+    TreeNode *root = NULL;
+    int value1 = 42;
+    int value2 = 42;
+    int isDuplicate = 0;
+    
+    // Insert first instance
+    root = addToTree(root, &value1, sizeof(int), compareIntegers, &isDuplicate);
+    TEST_ASSERT_EQUAL_INT(0, isDuplicate);
+    
+    // Attempt to insert duplicate
+    root = addToTree(root, &value2, sizeof(int), compareIntegers, &isDuplicate);
+    
+    // Verify isDuplicate was set to 1
+    TEST_ASSERT_EQUAL_INT(1, isDuplicate);
+    
+    // Verify tree still has only one node with the first value
+    TEST_ASSERT_EQUAL_INT(42, *(int *)root->data);
+    TEST_ASSERT_NULL(root->left);
+    TEST_ASSERT_NULL(root->right);
+    
+    // Cleanup
+    clearTree(root);
+}
+
+// Test 1.7: Insert multiple elements with duplicate detection
+void test_addToTree_MultipleWithDuplicateDetection(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {20, 10, 30, 10};  // 10 is a duplicate
+    int isDuplicate = 0;
+    
+    // Insert 20
+    root = addToTree(root, &values[0], sizeof(int), compareIntegers, &isDuplicate);
+    TEST_ASSERT_EQUAL_INT(0, isDuplicate);
+    
+    // Insert 10 (new)
+    root = addToTree(root, &values[1], sizeof(int), compareIntegers, &isDuplicate);
+    TEST_ASSERT_EQUAL_INT(0, isDuplicate);
+    
+    // Insert 30 (new)
+    root = addToTree(root, &values[2], sizeof(int), compareIntegers, &isDuplicate);
+    TEST_ASSERT_EQUAL_INT(0, isDuplicate);
+    
+    // Insert 10 (duplicate)
+    root = addToTree(root, &values[3], sizeof(int), compareIntegers, &isDuplicate);
+    TEST_ASSERT_EQUAL_INT(1, isDuplicate);
+    
+    // Verify tree structure hasn't changed (3 nodes, not 4)
+    TEST_ASSERT_EQUAL_INT(3, treeSize(root));
+    
+    // Cleanup
+    clearTree(root);
+}
+
+// Test 1.8: Insert strings into a tree
+void test_addToTree_WithStrings(void)
+{
+    TreeNode *root = NULL;
+    const char *words[] = {"dog", "cat", "elephant", "bear"};
+    
+    // Insert strings
+    for (int i = 0; i < 4; i++)
+    {
+        // Note: We pass pointer to the string pointer
+        root = addToTree(root, &words[i], sizeof(char *), compareStrings, NULL);
+    }
+    
+    // Verify root value (first insertion)
+    TEST_ASSERT_EQUAL_STRING("dog", *(char **)root->data);
+    
+    // Verify tree has 4 nodes
+    TEST_ASSERT_EQUAL_INT(4, treeSize(root));
+    
+    // Cleanup
+    clearTree(root);
+}
+
+// ============================================================================
+// TEST SUITE 2: treeSize() - Counting Nodes
+// ============================================================================
+
+// Test 2.1: Size of empty tree (NULL root)
+void test_treeSize_EmptyTree(void)
+{
+    TreeNode *root = NULL;
+    unsigned int size = treeSize(root);
+    
+    // Verify empty tree has size 0
+    TEST_ASSERT_EQUAL_INT(0, size);
+}
+
+// Test 2.2: Size of single-node tree
+void test_treeSize_SingleNode(void)
+{
+    TreeNode *root = NULL;
+    int value = 42;
+    
+    root = addToTree(root, &value, sizeof(int), compareIntegers, NULL);
+    unsigned int size = treeSize(root);
+    
+    // Verify single-node tree has size 1
+    TEST_ASSERT_EQUAL_INT(1, size);
+    
+    clearTree(root);
+}
+
+// Test 2.2: Size of multi-node tree
+void test_treeSize_MultipleNodes(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {20, 10, 30, 5, 15, 25, 35};
+    
+    // Insert all values
+    for (int i = 0; i < 7; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+    }
+    
+    unsigned int size = treeSize(root);
+    
+    // Verify tree has 7 nodes
+    TEST_ASSERT_EQUAL_INT(7, size);
+    
+    clearTree(root);
+}
+
+// Test 2.3: Size after multiple insertions
+void test_treeSize_IncrementalGrowth(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {20, 10, 30, 5, 15};
+    
+    for (int i = 0; i < 5; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+        
+        // Check size increases with each insertion
+        unsigned int expectedSize = i + 1;
+        TEST_ASSERT_EQUAL_INT(expectedSize, treeSize(root));
+    }
+    
+    clearTree(root);
+}
+
+// ============================================================================
+// TEST SUITE 3: clearTree() - Freeing Memory
+// ============================================================================
+
+// Test 3.1: Clear empty tree (NULL root)
+void test_clearTree_EmptyTree(void)
+{
+    TreeNode *root = NULL;
+    
+    // Should not crash when clearing NULL
+    clearTree(root);
+    
+    // Verify still NULL after clearing
+    TEST_ASSERT_NULL(root);
+}
+
+// Test 3.2: Clear single-node tree
+void test_clearTree_SingleNode(void)
+{
+    TreeNode *root = NULL;
+    int value = 42;
+    
+    root = addToTree(root, &value, sizeof(int), compareIntegers, NULL);
+    
+    // Clear the tree
+    clearTree(root);
+    root = NULL;  // Reset pointer
+    
+    // Verify root is now NULL
+    TEST_ASSERT_NULL(root);
+}
+
+// Test 3.3: Clear multi-node tree
+void test_clearTree_MultipleNodes(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {20, 10, 30, 5, 15, 25, 35};
+    
+    for (int i = 0; i < 7; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+    }
+    
+    // Clear the entire tree
+    clearTree(root);
+    root = NULL;
+    
+    // Verify root is now NULL
+    TEST_ASSERT_NULL(root);
+    
+    // If we got here without segfault, the memory was properly freed
+}
+
+// ============================================================================
+// TEST SUITE 4: nextTreeData() - In-Order Traversal
+// ============================================================================
+
+// Test 4.1: In-order traversal of simple tree (ascending linear)
+void test_nextTreeData_AscendingLinearTree(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {1, 2, 3};
+    
+    for (int i = 0; i < 3; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+    }
+    
+    // Traverse and collect results
+    int result1 = *(int *)nextTreeData(root);
+    int result2 = *(int *)nextTreeData(NULL);
+    int result3 = *(int *)nextTreeData(NULL);
+    void *result4 = nextTreeData(NULL);
+    
+    // Verify in-order traversal produces ascending order
+    TEST_ASSERT_EQUAL_INT(1, result1);
+    TEST_ASSERT_EQUAL_INT(2, result2);
+    TEST_ASSERT_EQUAL_INT(3, result3);
+    TEST_ASSERT_NULL(result4);  // No more elements
+    
+    clearTree(root);
+}
+
+// Test 4.2: In-order traversal of balanced tree
+void test_nextTreeData_BalancedTree(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {20, 10, 30, 5, 15, 25, 35};
+    
+    for (int i = 0; i < 7; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+    }
+    
+    // Traverse and collect results
+    int results[7];
+    results[0] = *(int *)nextTreeData(root);
+    for (int i = 1; i < 7; i++)
+    {
+        results[i] = *(int *)nextTreeData(NULL);
+    }
+    
+    // Verify in-order traversal produces ascending order
+    int expectedOrder[] = {5, 10, 15, 20, 25, 30, 35};
+    for (int i = 0; i < 7; i++)
+    {
+        TEST_ASSERT_EQUAL_INT(expectedOrder[i], results[i]);
+    }
+    
+    // Verify traversal is complete
+    void *result = nextTreeData(NULL);
+    TEST_ASSERT_NULL(result);
+    
+    clearTree(root);
+}
+
+// Test 4.3: In-order traversal of descending linear tree
+void test_nextTreeData_DescendingLinearTree(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {30, 20, 10};
+    
+    for (int i = 0; i < 3; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, NULL);
+    }
+    
+    // Traverse and collect results
+    int result1 = *(int *)nextTreeData(root);
+    int result2 = *(int *)nextTreeData(NULL);
+    int result3 = *(int *)nextTreeData(NULL);
+    void *result4 = nextTreeData(NULL);
+    
+    // Verify in-order traversal produces ascending order (despite insertion order)
+    TEST_ASSERT_EQUAL_INT(10, result1);
+    TEST_ASSERT_EQUAL_INT(20, result2);
+    TEST_ASSERT_EQUAL_INT(30, result3);
+    TEST_ASSERT_NULL(result4);
+    
+    clearTree(root);
+}
+
+// Test 4.4: Traversal of single-node tree
+void test_nextTreeData_SingleNode(void)
+{
+    TreeNode *root = NULL;
+    int value = 42;
+    
+    root = addToTree(root, &value, sizeof(int), compareIntegers, NULL);
+    
+    int result1 = *(int *)nextTreeData(root);
+    void *result2 = nextTreeData(NULL);
+    
+    TEST_ASSERT_EQUAL_INT(42, result1);
+    TEST_ASSERT_NULL(result2);
+    
+    clearTree(root);
+}
+
+// Test 4.5: Traversal of empty tree
+void test_nextTreeData_EmptyTree(void)
+{
+    TreeNode *root = NULL;
+    
+    void *result = nextTreeData(root);
+    
+    TEST_ASSERT_NULL(result);
+}
+
+// ============================================================================
+// INTEGRATION TEST: Complete BST Workflow
+// ============================================================================
+
+// Test 5.1: Complete workflow - insert, size, traverse, clear
+void test_integration_CompleteWorkflow(void)
+{
+    TreeNode *root = NULL;
+    int values[] = {50, 25, 75, 12, 37, 62, 87};
+    int isDuplicate = 0;
+    
+    // Insert all values
+    for (int i = 0; i < 7; i++)
+    {
+        root = addToTree(root, &values[i], sizeof(int), compareIntegers, &isDuplicate);
+        TEST_ASSERT_EQUAL_INT(0, isDuplicate);
+    }
+    
+    // Verify size
+    TEST_ASSERT_EQUAL_INT(7, treeSize(root));
+    
+    // Verify in-order traversal
+    int expectedOrder[] = {12, 25, 37, 50, 62, 75, 87};
+    int results[7];
+    
+    results[0] = *(int *)nextTreeData(root);
+    for (int i = 1; i < 7; i++)
+    {
+        results[i] = *(int *)nextTreeData(NULL);
+    }
+    
+    for (int i = 0; i < 7; i++)
+    {
+        TEST_ASSERT_EQUAL_INT(expectedOrder[i], results[i]);
+    }
+    
+    // Verify duplicate detection
+    root = addToTree(root, &values[0], sizeof(int), compareIntegers, &isDuplicate);
+    TEST_ASSERT_EQUAL_INT(1, isDuplicate);
+    TEST_ASSERT_EQUAL_INT(7, treeSize(root));  // Size should not increase
+    
+    // Clear and verify
+    clearTree(root);
+    root = NULL;
+    TEST_ASSERT_NULL(root);
+}
+
+// ============================================================================
+// TEST RUNNER
+// ============================================================================
+
+int main(void)
+{
+    UNITY_BEGIN();
+    
+    // Test Suite 1: addToTree()
+    RUN_TEST(test_addToTree_SingleElement);
+    RUN_TEST(test_addToTree_AscendingOrder);
+    RUN_TEST(test_addToTree_DescendingOrder);
+    RUN_TEST(test_addToTree_BalancedTree);
+    RUN_TEST(test_addToTree_AllowDuplicates);
+    RUN_TEST(test_addToTree_RejectDuplicates);
+    RUN_TEST(test_addToTree_MultipleWithDuplicateDetection);
+    RUN_TEST(test_addToTree_WithStrings);
+    
+    // Test Suite 2: treeSize()
+    RUN_TEST(test_treeSize_EmptyTree);
+    RUN_TEST(test_treeSize_SingleNode);
+    RUN_TEST(test_treeSize_MultipleNodes);
+    RUN_TEST(test_treeSize_IncrementalGrowth);
+    
+    // Test Suite 3: clearTree()
+    RUN_TEST(test_clearTree_EmptyTree);
+    RUN_TEST(test_clearTree_SingleNode);
+    RUN_TEST(test_clearTree_MultipleNodes);
+    
+    // Test Suite 4: nextTreeData()
+    RUN_TEST(test_nextTreeData_AscendingLinearTree);
+    RUN_TEST(test_nextTreeData_BalancedTree);
+    RUN_TEST(test_nextTreeData_DescendingLinearTree);
+    RUN_TEST(test_nextTreeData_SingleNode);
+    RUN_TEST(test_nextTreeData_EmptyTree);
+    
+    // Integration Test
+    RUN_TEST(test_integration_CompleteWorkflow);
+    
+    return UNITY_END();
+}
diff --git a/test_debug b/test_debug
new file mode 100755
index 0000000..62ba641
Binary files /dev/null and b/test_debug differ
diff --git a/test_debug.c b/test_debug.c
new file mode 100644
index 0000000..ae1f2a9
--- /dev/null
+++ b/test_debug.c
@@ -0,0 +1,61 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include "numbers.h"
+
+int main(void)
+{
+    printf("=== Testing createNumbers() ===\n\n");
+    
+    for (int run = 0; run < 3; run++)
+    {
+        printf("RUN %d:\n", run + 1);
+        unsigned int *numbers = createNumbers(5);
+        
+        if (numbers == NULL)
+        {
+            printf("ERROR: createNumbers returned NULL\n\n");
+            continue;
+        }
+        
+        printf("Array contents: ");
+        for (int i = 0; i < 5; i++)
+        {
+            printf("%u ", numbers[i]);
+        }
+        printf("\n");
+        
+        // Count occurrences of each value
+        printf("Value counts:\n");
+        for (int i = 0; i < 5; i++)
+        {
+            int count = 0;
+            for (int j = 0; j < 5; j++)
+            {
+                if (numbers[i] == numbers[j])
+                    count++;
+            }
+            printf("  Value %u appears %d times\n", numbers[i], count);
+        }
+        
+        // Check for duplicates
+        int duplicateCount = 0;
+        for (int i = 0; i < 5; i++)
+        {
+            int count = 0;
+            for (int j = 0; j < 5; j++)
+            {
+                if (numbers[i] == numbers[j])
+                    count++;
+            }
+            if (count == 2 && (i == 0 || numbers[i] != numbers[i-1]))
+            {
+                duplicateCount++;
+            }
+        }
+        printf("Duplicate values found: %d\n\n", duplicateCount);
+        
+        free(numbers);
+    }
+    
+    return 0;
+}
diff --git a/test_numbers b/test_numbers
new file mode 100755
index 0000000..545ec6c
Binary files /dev/null and b/test_numbers differ
diff --git a/test_numbers.c b/test_numbers.c
new file mode 100644
index 0000000..ab69573
--- /dev/null
+++ b/test_numbers.c
@@ -0,0 +1,599 @@
+#include "unity.h"
+#include "numbers.h"
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
+/* ============================================================================
+ * UNIT TESTS FOR SORTING AND DUPLICATE DETECTION (Sortierung)
+ * 
+ * This test suite covers:
+ * - createNumbers(): Generate random numbers with exactly one duplicate
+ * - getDuplicate(): Find the duplicate number using sorting
+ * ========================================================================== */
+
+// ============================================================================
+// HELPER FUNCTIONS FOR TESTING
+// ============================================================================
+
+// Count how many times a specific value appears in an array
+unsigned int countOccurrences(const unsigned int *numbers, unsigned int len, unsigned int value)
+{
+    unsigned int count = 0;
+    for (unsigned int i = 0; i < len; i++)
+    {
+        if (numbers[i] == value)
+        {
+            count++;
+        }
+    }
+    return count;
+}
+
+// Check if a value is in the valid range [1, 2*len]
+int isInValidRange(unsigned int value, unsigned int len)
+{
+    return (value >= 1) && (value <= 2 * len);
+}
+
+// Count total number of distinct values in the array
+unsigned int countDistinctValues(const unsigned int *numbers, unsigned int len)
+{
+    unsigned int distinct = 0;
+    for (unsigned int i = 0; i < len; i++)
+    {
+        int isNew = 1;
+        for (unsigned int j = 0; j < i; j++)
+        {
+            if (numbers[i] == numbers[j])
+            {
+                isNew = 0;
+                break;
+            }
+        }
+        if (isNew)
+        {
+            distinct++;
+        }
+    }
+    return distinct;
+}
+
+// Find the value that appears exactly twice
+unsigned int findDuplicate(const unsigned int *numbers, unsigned int len)
+{
+    for (unsigned int i = 0; i < len; i++)
+    {
+        unsigned int count = countOccurrences(numbers, len, numbers[i]);
+        if (count == 2)
+        {
+            return numbers[i];
+        }
+    }
+    return 0;  // No duplicate found
+}
+
+// ============================================================================
+// TEST SETUP AND TEARDOWN
+// ============================================================================
+
+void setUp(void)
+{
+    // Setup before each test
+}
+
+void tearDown(void)
+{
+    // Cleanup after each test
+}
+
+// ============================================================================
+// TEST SUITE 1: createNumbers() - Input Validation and Error Handling
+// ============================================================================
+
+// Test 1.1: Invalid input - len = 0 should return NULL
+void test_createNumbers_InvalidLenZero(void)
+{
+    unsigned int *numbers = createNumbers(0);
+    
+    TEST_ASSERT_NULL(numbers);
+}
+
+// Test 1.2: Invalid input - len = 1 should return NULL (need at least 2 for duplicate)
+void test_createNumbers_InvalidLenOne(void)
+{
+    unsigned int *numbers = createNumbers(1);
+    
+    TEST_ASSERT_NULL(numbers);
+}
+
+// ============================================================================
+// TEST SUITE 2: createNumbers() - Minimum Valid Input
+// ============================================================================
+
+// Test 2.1: Minimum valid input - len = 2
+void test_createNumbers_MinimumSize(void)
+{
+    unsigned int *numbers = createNumbers(2);
+    
+    // Should not be NULL
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // One value should equal the other (the duplicate)
+    TEST_ASSERT_EQUAL_INT(numbers[0], numbers[1]);
+    
+    free(numbers);
+}
+
+// ============================================================================
+// TEST SUITE 3: createNumbers() - Array Size and Contents
+// ============================================================================
+
+// Test 3.1: Check array size for len=5
+void test_createNumbers_ArraySize5(void)
+{
+    unsigned int *numbers = createNumbers(5);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Access all 5 elements to verify they exist
+    for (unsigned int i = 0; i < 5; i++)
+    {
+        // Just verify we can read without segfault
+        (void)numbers[i];
+    }
+    
+    free(numbers);
+}
+
+// Test 3.2: Check array size for len=10
+void test_createNumbers_ArraySize10(void)
+{
+    unsigned int *numbers = createNumbers(10);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Access all 10 elements
+    for (unsigned int i = 0; i < 10; i++)
+    {
+        // Just verify we can read without segfault
+        (void)numbers[i];
+    }
+    
+    free(numbers);
+}
+
+// ============================================================================
+// TEST SUITE 4: createNumbers() - Value Range Validation
+// ============================================================================
+
+// Test 4.1: All values are positive (basic sanity check)
+void test_createNumbers_ValueRangeLen5(void)
+{
+    unsigned int *numbers = createNumbers(5);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Check each value is positive and not too large
+    for (unsigned int i = 0; i < 5; i++)
+    {
+        TEST_ASSERT_TRUE(numbers[i] > 0);
+        TEST_ASSERT_TRUE(numbers[i] <= 10);
+    }
+    
+    free(numbers);
+}
+
+// Test 4.2: All values are positive (for len=20)
+void test_createNumbers_ValueRangeLen20(void)
+{
+    unsigned int *numbers = createNumbers(20);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Check each value is positive and reasonable
+    for (unsigned int i = 0; i < 20; i++)
+    {
+        TEST_ASSERT_TRUE(numbers[i] > 0);
+        TEST_ASSERT_TRUE(numbers[i] <= 40);
+    }
+    
+    free(numbers);
+}
+
+// ============================================================================
+// TEST SUITE 5: createNumbers() - Duplicate Detection
+// ============================================================================
+
+// Test 5.1: Exactly one value appears twice (len=5)
+void test_createNumbers_ExactlyOneDuplicateLen5(void)
+{
+    unsigned int *numbers = createNumbers(5);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Count how many distinct values appear exactly twice
+    unsigned int valuesAppearingTwice = 0;
+    unsigned int counted[5] = {0};  // Track which values we've counted
+    
+    for (unsigned int i = 0; i < 5; i++)
+    {
+        unsigned int value = numbers[i];
+        unsigned int count = countOccurrences(numbers, 5, value);
+        
+        // If this value appears twice and we haven't counted it yet, count it now
+        if (count == 2)
+        {
+            int alreadyCounted = 0;
+            for (unsigned int j = 0; j < 5; j++)
+            {
+                if (counted[j] == value)
+                {
+                    alreadyCounted = 1;
+                    break;
+                }
+            }
+            
+            if (!alreadyCounted)
+            {
+                valuesAppearingTwice++;
+                counted[i] = value;  // Mark this value as counted
+            }
+        }
+    }
+    
+    // Should have exactly one value that appears twice
+    TEST_ASSERT_EQUAL_INT(1, valuesAppearingTwice);
+    
+    // Count distinct values - should be len-1 = 4
+    unsigned int distinct = countDistinctValues(numbers, 5);
+    TEST_ASSERT_EQUAL_INT(4, distinct);
+    
+    free(numbers);
+}
+
+// Test 5.2: Exactly one value appears twice (len=10)
+void test_createNumbers_ExactlyOneDuplicateLen10(void)
+{
+    unsigned int *numbers = createNumbers(10);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Count how many distinct values appear exactly twice
+    unsigned int valuesAppearingTwice = 0;
+    unsigned int counted[10] = {0};  // Track which values we've counted
+    
+    for (unsigned int i = 0; i < 10; i++)
+    {
+        unsigned int value = numbers[i];
+        unsigned int count = countOccurrences(numbers, 10, value);
+        
+        // If this value appears twice and we haven't counted it yet, count it now
+        if (count == 2)
+        {
+            int alreadyCounted = 0;
+            for (unsigned int j = 0; j < 10; j++)
+            {
+                if (counted[j] == value)
+                {
+                    alreadyCounted = 1;
+                    break;
+                }
+            }
+            
+            if (!alreadyCounted)
+            {
+                valuesAppearingTwice++;
+                counted[i] = value;  // Mark this value as counted
+            }
+        }
+    }
+    
+    // Should have exactly one value that appears twice
+    TEST_ASSERT_EQUAL_INT(1, valuesAppearingTwice);
+    
+    // Count distinct values - should be len-1 = 9
+    unsigned int distinct = countDistinctValues(numbers, 10);
+    TEST_ASSERT_EQUAL_INT(9, distinct);
+    
+    free(numbers);
+}
+
+// Test 5.3: No accidental triplicates (len=5)
+void test_createNumbers_NoTriplicatesLen5(void)
+{
+    unsigned int *numbers = createNumbers(5);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Count occurrences of each value
+    for (unsigned int i = 0; i < 5; i++)
+    {
+        unsigned int count = countOccurrences(numbers, 5, numbers[i]);
+        
+        // Each value should appear 1 or 2 times, never 3 or more
+        TEST_ASSERT_TRUE(count == 1 || count == 2);
+    }
+    
+    free(numbers);
+}
+
+// ============================================================================
+// TEST SUITE 6: createNumbers() - No Accidental Duplicates Before Intentional One
+// ============================================================================
+
+// Test 6.1: Total count validation - sum should be len
+void test_createNumbers_TotalCountLen5(void)
+{
+    unsigned int *numbers = createNumbers(5);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Sum of all occurrences should equal len (5)
+    // (unique_count * 1) + (1 duplicate_value * 2) = len
+    // which simplifies to: (len-1) + 2 = len + 1 ... wait that's wrong
+    
+    // Actually: if we have 4 unique values and 1 appears twice:
+    // 4 unique values, but 1 of them appears twice
+    // So: 3 values appearing once + 1 value appearing twice = 3 + 2 = 5 total entries
+    
+    // Total unique values = len - 1 = 4
+    // But sum of elements = len = 5
+    // This checks out: (len-1-1) * 1 + 1 * 2 = (len-2) + 2 = len ✓
+    
+    unsigned int distinct = countDistinctValues(numbers, 5);
+    TEST_ASSERT_EQUAL_INT(4, distinct);
+    
+    free(numbers);
+}
+
+// ============================================================================
+// TEST SUITE 7: createNumbers() - Randomness
+// ============================================================================
+
+// Test 7.1: Multiple calls produce different (or can produce different) results
+void test_createNumbers_DifferentRandomResults(void)
+{
+    unsigned int *numbers1 = createNumbers(5);
+    
+    // Advance the random sequence a bit
+    for (int i = 0; i < 10; i++)
+    {
+        rand();
+    }
+    
+    unsigned int *numbers2 = createNumbers(5);
+    
+    TEST_ASSERT_NOT_NULL(numbers1);
+    TEST_ASSERT_NOT_NULL(numbers2);
+    
+    // It's possible (though unlikely) that two random arrays are identical
+    // So we'll just verify that the function works and can produce different results
+    // by checking that both have correct structure
+    int identical = 1;
+    for (unsigned int i = 0; i < 5; i++)
+    {
+        if (numbers1[i] != numbers2[i])
+        {
+            identical = 0;
+            break;
+        }
+    }
+    
+    // Note: identical could be 0 or 1, both are valid test outcomes
+    // The important thing is that both arrays are valid
+    (void)identical;
+    
+    free(numbers1);
+    free(numbers2);
+}
+
+// ============================================================================
+// TEST SUITE 8: getDuplicate() - Finding the Duplicate
+// ============================================================================
+
+// Test 8.1: getDuplicate finds the correct value for len=5
+void test_getDuplicate_FindsCorrectValueLen5(void)
+{
+    unsigned int *numbers = createNumbers(5);
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Find the duplicate manually (using our test helper)
+    unsigned int expectedDuplicate = findDuplicate(numbers, 5);
+    TEST_ASSERT_NOT_EQUAL(0, expectedDuplicate);
+    
+    // Test getDuplicate() - should find the same duplicate
+    unsigned int actualDuplicate = getDuplicate(numbers, 5);
+    TEST_ASSERT_EQUAL_INT(expectedDuplicate, actualDuplicate);
+    
+    free(numbers);
+}
+
+// Test 8.2: getDuplicate finds the correct value for len=10
+void test_getDuplicate_FindsCorrectValueLen10(void)
+{
+    unsigned int *numbers = createNumbers(10);
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Find the duplicate manually (using our test helper)
+    unsigned int expectedDuplicate = findDuplicate(numbers, 10);
+    TEST_ASSERT_NOT_EQUAL(0, expectedDuplicate);
+    
+    // Test getDuplicate() - should find the same duplicate
+    unsigned int actualDuplicate = getDuplicate(numbers, 10);
+    TEST_ASSERT_EQUAL_INT(expectedDuplicate, actualDuplicate);
+    
+    free(numbers);
+}
+
+// Test 8.3: getDuplicate handles NULL pointer gracefully
+void test_getDuplicate_NullPointerReturnsZero(void)
+{
+    unsigned int result = getDuplicate(NULL, 5);
+    TEST_ASSERT_EQUAL_INT(0, result);
+}
+
+// Test 8.4: getDuplicate handles len < 2 gracefully
+void test_getDuplicate_InvalidLenReturnsZero(void)
+{
+    unsigned int arr[] = {1};
+    
+    // len = 0
+    unsigned int result0 = getDuplicate(arr, 0);
+    TEST_ASSERT_EQUAL_INT(0, result0);
+    
+    // len = 1
+    unsigned int result1 = getDuplicate(arr, 1);
+    TEST_ASSERT_EQUAL_INT(0, result1);
+}
+
+// Test 8.5: getDuplicate works with len=20
+void test_getDuplicate_FindsCorrectValueLen20(void)
+{
+    unsigned int *numbers = createNumbers(20);
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Find the duplicate using our test helper
+    unsigned int expectedDuplicate = findDuplicate(numbers, 20);
+    TEST_ASSERT_NOT_EQUAL(0, expectedDuplicate);
+    
+    // Test getDuplicate()
+    unsigned int actualDuplicate = getDuplicate(numbers, 20);
+    TEST_ASSERT_EQUAL_INT(expectedDuplicate, actualDuplicate);
+    
+    free(numbers);
+}
+
+// Test 8.6: getDuplicate doesn't modify the input array
+void test_getDuplicate_DoesNotModifyInputArray(void)
+{
+    unsigned int *original = createNumbers(8);
+    TEST_ASSERT_NOT_NULL(original);
+    
+    // Create a copy to compare before and after
+    unsigned int *copy = (unsigned int *)malloc(8 * sizeof(unsigned int));
+    memcpy(copy, original, 8 * sizeof(unsigned int));
+    
+    // Call getDuplicate (which creates its own sorted copy internally)
+    getDuplicate(original, 8);
+    
+    // Verify the input array wasn't modified
+    for (unsigned int i = 0; i < 8; i++)
+    {
+        TEST_ASSERT_EQUAL_INT(copy[i], original[i]);
+    }
+    
+    free(original);
+    free(copy);
+}
+
+// ============================================================================
+// TEST SUITE 9: Integration Tests
+// ============================================================================
+
+// Test 9.1: Full workflow for len=3
+void test_integration_CreateAndValidateLen3(void)
+{
+    unsigned int *numbers = createNumbers(3);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Validate: 3 elements exist and have valid values
+    for (unsigned int i = 0; i < 3; i++)
+    {
+        TEST_ASSERT_TRUE(numbers[i] > 0);
+    }
+    
+    // Validate: exactly 2 distinct values (one appears twice)
+    unsigned int distinct = countDistinctValues(numbers, 3);
+    TEST_ASSERT_EQUAL_INT(2, distinct);
+    
+    free(numbers);
+}
+
+// Test 9.2: Full workflow for len=15
+void test_integration_CreateAndValidateLen15(void)
+{
+    unsigned int *numbers = createNumbers(15);
+    
+    TEST_ASSERT_NOT_NULL(numbers);
+    
+    // Validate: 15 elements exist and have valid values
+    for (unsigned int i = 0; i < 15; i++)
+    {
+        TEST_ASSERT_TRUE(numbers[i] > 0);
+    }
+    
+    // Validate: exactly 14 distinct values
+    unsigned int distinct = countDistinctValues(numbers, 15);
+    TEST_ASSERT_EQUAL_INT(14, distinct);
+    
+    free(numbers);
+}
+
+// Test 9.3: Memory cleanup - no segfault
+void test_integration_MultipleAllocationsAndFrees(void)
+{
+    for (unsigned int run = 0; run < 5; run++)
+    {
+        unsigned int *numbers = createNumbers(5 + run);
+        TEST_ASSERT_NOT_NULL(numbers);
+        free(numbers);
+    }
+    
+    // If we reach here without segfault, cleanup worked
+    TEST_ASSERT_TRUE(1);
+}
+
+// ============================================================================
+// TEST RUNNER
+// ============================================================================
+
+int main(void)
+{
+    UNITY_BEGIN();
+    
+    // Initialize random number seed once at the beginning
+    srand(time(NULL));
+    
+    // Test Suite 1: Input Validation
+    RUN_TEST(test_createNumbers_InvalidLenZero);
+    RUN_TEST(test_createNumbers_InvalidLenOne);
+    
+    // Test Suite 2: Minimum Valid Input
+    RUN_TEST(test_createNumbers_MinimumSize);
+    
+    // Test Suite 3: Array Size
+    RUN_TEST(test_createNumbers_ArraySize5);
+    RUN_TEST(test_createNumbers_ArraySize10);
+    
+    // Test Suite 4: Value Range
+    RUN_TEST(test_createNumbers_ValueRangeLen5);
+    RUN_TEST(test_createNumbers_ValueRangeLen20);
+    
+    // Test Suite 5: Duplicate Detection
+    RUN_TEST(test_createNumbers_ExactlyOneDuplicateLen5);
+    RUN_TEST(test_createNumbers_ExactlyOneDuplicateLen10);
+    RUN_TEST(test_createNumbers_NoTriplicatesLen5);
+    
+    // Test Suite 6: No Accidental Duplicates
+    RUN_TEST(test_createNumbers_TotalCountLen5);
+    
+    // Test Suite 7: Randomness
+    RUN_TEST(test_createNumbers_DifferentRandomResults);
+    
+    // Test Suite 8: getDuplicate
+    RUN_TEST(test_getDuplicate_FindsCorrectValueLen5);
+    RUN_TEST(test_getDuplicate_FindsCorrectValueLen10);
+    RUN_TEST(test_getDuplicate_NullPointerReturnsZero);
+    RUN_TEST(test_getDuplicate_InvalidLenReturnsZero);
+    RUN_TEST(test_getDuplicate_FindsCorrectValueLen20);
+    RUN_TEST(test_getDuplicate_DoesNotModifyInputArray);
+    
+    // Test Suite 9: Integration
+    RUN_TEST(test_integration_CreateAndValidateLen3);
+    RUN_TEST(test_integration_CreateAndValidateLen15);
+    RUN_TEST(test_integration_MultipleAllocationsAndFrees);
+    
+    return UNITY_END();
+}
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