diff --git a/bintree.c b/bintree.c
index 5cf82a9..1686c2b 100644
--- a/bintree.c
+++ b/bintree.c
@@ -1,36 +1,117 @@
 #include <string.h>
+#include <stdlib.h>
 #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. */
+static StackNode *iterStack = NULL;
+static void pushLeftBranch(StackNode **stack, TreeNode *node);
 
-// 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)
+// Inserts a new node into the BST.
+// If isDuplicate == NULL → duplicates are allowed
+// If isDuplicate != NULL → duplicates are ignored and *isDuplicate = 1
+TreeNode *addToTree(TreeNode *root, const void *data, size_t dataSize,
+                    CompareFctType compareFct, int *isDuplicate)
 {
+    if (root == NULL)
+    {
+        TreeNode *newNode = calloc(1, sizeof(TreeNode));
+        if (!newNode)
+            return NULL;
 
+        newNode->data = malloc(dataSize);
+        if (!newNode->data)
+        {
+            free(newNode);
+            return NULL;
+        }
+
+        memcpy(newNode->data, data, dataSize);
+
+        if (isDuplicate)
+            *isDuplicate = 0;
+
+        return newNode;
+    }
+
+    int cmp = compareFct(data, root->data);
+
+    if (cmp < 0 || (cmp == 0 && isDuplicate == NULL))
+    {
+        root->left = addToTree(root->left, data, dataSize, compareFct, isDuplicate);
+    }
+    else if (cmp > 0)
+    {
+        root->right = addToTree(root->right, data, dataSize, compareFct, isDuplicate);
+    }
+    else
+    {
+        if (isDuplicate)
+            *isDuplicate = 1;
+    }
+
+    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.
+static void pushLeftBranch(StackNode **stack, TreeNode *node)
+{
+    while (node)
+    {
+        *stack = push(*stack, node);
+        node = node->left;
+    }
+}
+
+// If root != NULL → reset iterator and start from new tree.
+// If root == NULL → continue iterating.
 void *nextTreeData(TreeNode *root)
 {
+    // Start new iteration
+    if (root != NULL)
+    {
+        // reset old iterator state
+        clearStack(iterStack);
+        iterStack = NULL;
 
+        // push root and all left children
+        pushLeftBranch(&iterStack, root);
+    }
+
+    // No active iterator
+    if (iterStack == NULL)
+        return NULL;
+
+    // Get next node
+    TreeNode *node = (TreeNode *)top(iterStack);
+    iterStack = pop(iterStack);
+
+    // push right subtree and its left descendants
+    if (node->right)
+        pushLeftBranch(&iterStack, node->right);
+
+    return node->data;
 }
 
-// Releases all memory resources (including data copies).
+// Frees all nodes and also resets iterator.
 void clearTree(TreeNode *root)
 {
+    if (!root)
+        return;
 
+    clearTree(root->left);
+    clearTree(root->right);
+
+    free(root->data);
+    free(root);
+
+    // If we clear the tree, iterator must not point into freed memory.
+    clearStack(iterStack);
+    iterStack = NULL;
 }
 
-// Returns the number of entries in the tree given by root.
 unsigned int treeSize(const TreeNode *root)
 {
+    if (!root)
+        return 0;
 
-}
\ No newline at end of file
+    return 1 + treeSize(root->left) + treeSize(root->right);
+}