diff --git a/bintree.c b/bintree.c
index b73a4c8..cd4571a 100644
--- a/bintree.c
+++ b/bintree.c
@@ -76,7 +76,85 @@ 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)
 {
+/* static iterator state */
+    static TreeNode **stack = NULL;
+    static int top = -1;
+    static int capacity = 0;
+    static TreeNode *currentRoot = NULL;
 
+    /* helper: ensure capacity */
+    if (capacity == 0) {
+        capacity = 16;
+        stack = (TreeNode**)malloc(capacity * sizeof(TreeNode*));
+        if (stack == NULL) { capacity = 0; return NULL; }
+    }
+
+    /* helper: push one node */
+    #define PUSH_NODE(n) do { \
+        if (top + 1 >= capacity) { \
+            int newcap = capacity * 2; \
+            TreeNode **tmp = (TreeNode**)realloc(stack, newcap * sizeof(TreeNode*)); \
+            if (tmp == NULL) { /* allocation failed, keep old stack */ break; } \
+            stack = tmp; capacity = newcap; \
+        } \
+        stack[++top] = (n); \
+    } while(0)
+
+    /* helper: push node and all left descendants */
+    void push_lefts(TreeNode *n) {
+        while (n != NULL) {
+            PUSH_NODE(n);
+            n = n->left;
+        }
+    }
+
+    /* If a new root is provided and differs from current, reset iterator */
+    if (root != NULL && root != currentRoot) {
+        free(stack);
+        stack = NULL;
+        top = -1;
+        capacity = 0;
+        currentRoot = root;
+
+        /* reinitialize stack */
+        capacity = 16;
+        stack = (TreeNode**)malloc(capacity * sizeof(TreeNode*));
+        if (stack == NULL) { capacity = 0; return NULL; }
+        top = -1;
+        push_lefts(root);
+    } else if (root != NULL && root == currentRoot) {
+        /* explicit restart with same root: reset and push again */
+        free(stack);
+        stack = NULL;
+        top = -1;
+        capacity = 0;
+        capacity = 16;
+        stack = (TreeNode**)malloc(capacity * sizeof(TreeNode*));
+        if (stack == NULL) { capacity = 0; return NULL; }
+        top = -1;
+        push_lefts(root);
+    }
+    /* if root == NULL: continue with existing stack */
+
+    /* nothing left */
+    if (top < 0) {
+        free(stack);
+        stack = NULL;
+        capacity = 0;
+        currentRoot = NULL;
+        return NULL;
+    }
+
+    /* pop top */
+    TreeNode *node = stack[top--];
+    void *result = node->data;
+
+    /* if right child exists, push it and all its lefts */
+    if (node->right != NULL) {
+        push_lefts(node->right);
+    }
+
+    return result;
 }
 
 // Releases all memory resources (including data copies).