diff --git a/bintree.c b/bintree.c
index 5cf82a9..4cecb66 100644
--- a/bintree.c
+++ b/bintree.c
@@ -1,18 +1,82 @@
 #include <string.h>
 #include "stack.h"
 #include "bintree.h"
+#include <stdlib.h>
 
-//TODO: binären Suchbaum implementieren
+// 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. */
+ * `clearTree`: gibt den gesamten Baum frei (rekursiv),
+ * `treeSize`: zählt die Knoten im Baum (rekursiv),
+ * `nextTreeData`: Traversierung mit Hilfe des zuvor implementierten Stacks. */
 
 // 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 *insertedNode;
 
+    // create a new node if the current node is NULL
+    if (root == NULL)
+    {
+        // it's important to zero the pointers for adjacent nodes
+        insertedNode = calloc(1, sizeof(TreeNode));
+        if (!insertedNode)
+        {
+            return NULL;
+        }
+
+        insertedNode->data = malloc(dataSize);
+        if (!insertedNode->data)
+        {
+            return NULL;
+        }
+        memcpy(insertedNode->data, data, dataSize);
+        // reset isDuplicate if it exists
+        if (isDuplicate)
+        {
+            *isDuplicate = 0;
+        }
+        return insertedNode;
+    }
+
+    // TODO: what is the correct data type here?
+    int cmpRes = (*compareFct)(data, root->data);
+    // insert into the left branch
+    if (cmpRes < 0 || (cmpRes == 0 && isDuplicate == NULL))
+    {
+        root->left = addToTree(root->left, data, dataSize, compareFct, isDuplicate);
+    }
+    // insert into the right branch
+    else if (cmpRes > 0)
+    {
+        root->right = addToTree(root->right, data, dataSize, compareFct, isDuplicate);
+    }
+    // the data is equal to the current node
+    else
+    {
+        // the data already exists in the tree and duplicates are ignored (isDuplicate* not NULL)
+        *isDuplicate = 1;
+    }
+    return root;
+}
+
+// push all left descendants from @param node
+static void pushLeftDesc(StackNode **stackPtr, TreeNode *node)
+{
+    if (!stackPtr || !node)
+    {
+        return;
+    }
+    TreeNode *curNode = node;
+    while (curNode->left)
+    {
+        *stackPtr = push(*stackPtr, curNode->left);
+        if (!*stackPtr)
+        {
+            return;
+        }
+        curNode = curNode->left;
+    }
 }
 
 // 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.
@@ -20,17 +84,72 @@ TreeNode *addToTree(TreeNode *root, const void *data, size_t dataSize, CompareFc
 // push the top node and push all its left nodes.
 void *nextTreeData(TreeNode *root)
 {
+    // this creates a static variable that maintains an internal state
+    static StackNode *stack;
 
+    // create a new stack
+    if (root)
+    {
+        // clear possibly existing stacks
+        clearStack(stack);
+        // init a new stack
+        stack = push(NULL, root);
+        // init failed
+        if (!stack)
+        {
+            return NULL;
+        }
+        pushLeftDesc(&stack, root);
+
+        // return the first val
+        return nextTreeData(NULL);
+    }
+
+    // neither stack nor root exist
+    if (!stack)
+    {
+        return NULL;
+    }
+
+    // get next val with stack
+    TreeNode *res = top(stack);
+    stack = pop(stack);
+    if (res->right)
+    {
+        stack = push(stack, res->right);
+        pushLeftDesc(&stack, res->right);
+    }
+
+    return res->data;
 }
 
 // Releases all memory resources (including data copies).
 void clearTree(TreeNode *root)
 {
+    // this check is crucial for recursion
+    if (!root)
+    {
+        // nothing to clear
+        return;
+    }
 
+    // release the resources of child nodes first
+    clearTree(root->left);
+    clearTree(root->right);
+
+    // free the data (it's just a copy created in addToTree())
+    free(root->data);
+    free(root);
 }
 
 // Returns the number of entries in the tree given by root.
 unsigned int treeSize(const TreeNode *root)
 {
+    // there are no nodes
+    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 9387524..0a48b4d 100644
--- a/numbers.c
+++ b/numbers.c
@@ -18,17 +18,23 @@ unsigned int *createNumbers(unsigned int len)
 
   for (unsigned int i = 0; i < len; i++) {
     do
-    { 
+    {
+      isDuplicate = 0;
       numbers[i] = rand () % upperLimit + 1;
       binTree = addToTree(binTree, &numbers[i], sizeof(unsigned int), compare, &isDuplicate);
     } while (isDuplicate);
   } 
 
   unsigned int duplicate = numbers[rand () % len];
-  int indexDuplicate = rand () % len;
-  numbers[len] = numbers[indexDuplicate];
-  numbers[indexDuplicate] = duplicate;
+  int indexDuplicate;
 
+  do {
+    indexDuplicate = rand() % len;
+  } while (numbers[indexDuplicate] == duplicate);
+
+  numbers[len-1] = numbers[indexDuplicate];
+  numbers[indexDuplicate] = duplicate;
+  
   return numbers;
 }