diff --git a/bintree.c b/bintree.c
index 5cf82a9..a9c8211 100644
--- a/bintree.c
+++ b/bintree.c
@@ -10,27 +10,98 @@
 
 // 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).
-TreeNode *addToTree(TreeNode *root, const void *data, size_t dataSize, CompareFctType compareFct, int *isDuplicate)
-{
+TreeNode *addToTree(TreeNode *root,
+                    const void *data,
+                    size_t dataSize,
+                    CompareFctType compareFct,
+                    int *isDuplicate) {
+    if (!root) {
+        // allocate new node
+        TreeNode *node = malloc(sizeof(TreeNode));
+        if (!node) return NULL;
 
+        node->data = malloc(dataSize);
+        if (!node->data) {
+            free(node);
+            return NULL;
+        }
+
+        memcpy(node->data, data, dataSize);
+        node->left = node->right = NULL;
+
+        if (isDuplicate) *isDuplicate = 0;
+        return node;
+    }
+
+    int cmp = compareFct(data, root->data);
+
+    if (cmp == 0) {
+        // duplicate
+        if (isDuplicate) *isDuplicate = 1;
+        return root; // do not insert duplicate
+    }
+
+    if (cmp < 0)
+        root->left = addToTree(root->left, data, dataSize, compareFct, isDuplicate);
+    else
+        root->right = addToTree(root->right, data, dataSize, compareFct, isDuplicate);
+
+    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.
 void *nextTreeData(TreeNode *root)
 {
+    static StackNode *stack = NULL;
 
+    // New tree → reset stack and push left path
+    if (root != NULL) {
+        clearStack(stack);
+        stack = NULL;
+
+        TreeNode *curr = root;
+        while (curr) {
+            stack = push(stack, curr);
+            curr = curr->left;
+        }
+    }
+
+    // No tree / finished
+    if (!stack) return NULL;
+
+    // Top node
+    TreeNode *node = top(stack);
+    stack = pop(stack);
+
+    // After returning node, push its right subtree (all left nodes)
+    TreeNode *curr = node->right;
+    while (curr) {
+        stack = push(stack, curr);
+        curr = curr->left;
+    }
+
+    return node->data;
 }
 
 // Releases all memory resources (including data copies).
 void clearTree(TreeNode *root)
 {
+    if (!root) return;
 
+    clearTree(root->left);
+    clearTree(root->right);
+
+    free(root->data);
+    free(root);
 }
 
+
 // Returns the number of entries in the tree given by root.
 unsigned int treeSize(const TreeNode *root)
 {
-
+    if (!root) return 0;
+    return 1 + treeSize(root->left) + treeSize(root->right);
 }
\ No newline at end of file
diff --git a/numbers.c b/numbers.c
index f59d9a2..66bc744 100644
--- a/numbers.c
+++ b/numbers.c
@@ -16,11 +16,75 @@
 // creating random numbers.
 unsigned int *createNumbers(unsigned int len)
 {
+  if (len < 2) return NULL;
 
+  unsigned int *numbers = malloc(len * sizeof(unsigned int));
+  if (!numbers) return NULL;
+
+  srand((unsigned int)time(NULL));
+
+  TreeNode *root = NULL;
+  unsigned int maxValue = 2 * len;
+
+  unsigned int uniqueCount = len - 1;  // we generate len-1 unique values
+  unsigned int i = 0;
+
+  // Fill len - 1 unique values using BST
+  while (i < uniqueCount) {
+    unsigned int r = (rand() % maxValue) + 1;
+    int isDup = 0;
+
+    root = addToTree(root, &r, sizeof(unsigned int), compareUnsigned, &isDup);
+
+    if (!isDup) {
+      numbers[i] = r;
+      i++;
+    }
+  }
+
+  // Pick one number already in the list → duplicate it
+  unsigned int duplicateIndex = rand() % uniqueCount;
+  unsigned int duplicateValue = numbers[duplicateIndex];
+
+  numbers[len - 1] = duplicateValue;
+
+  // Cleanup tree
+  clearTree(root);
+
+  // Shuffle array for randomness
+  for (unsigned int j = 0; j < len; j++) {
+    unsigned int k = rand() % len;
+    unsigned int tmp = numbers[j];
+    numbers[j] = numbers[k];
+    numbers[k] = tmp;
+  }
+
+  return numbers;
 }
 
+
 // 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)
 {
+  if (!numbers || len < 2) return 0;
 
-}
\ No newline at end of file
+  // Make a copy because qsort modifies the array
+  unsigned int *copy = malloc(len * sizeof(unsigned int));
+  if (!copy) return 0;
+
+  memcpy(copy, numbers, len * sizeof(unsigned int));
+
+  qsort(copy, len, sizeof(unsigned int), compareUnsigned);
+
+  // Check adjacent elements for equality
+  for (unsigned int i = 0; i < len - 1; i++) {
+    if (copy[i] == copy[i + 1]) {
+      unsigned int result = copy[i];
+      free(copy);
+      return result;
+    }
+  }
+
+  free(copy);
+  return 0; // no duplicate found (should not happen)
+}