diff --git a/bintree.c b/bintree.c
index b194df7..59d0c46 100644
--- a/bintree.c
+++ b/bintree.c
@@ -56,7 +56,34 @@ 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)
 {
+    /* Iterator state kept across calls (strtok-style). */
+    static StackNode *iterStack = NULL;
 
+    /* Starting a new traversal: reset stack and seed with root's left spine. */
+    if (root != NULL) {
+        clearStack(iterStack);
+        iterStack = NULL;
+
+        for (TreeNode *n = root; n != NULL; n = n->left) {
+            iterStack = push(iterStack, n);
+        }
+    }
+
+    /* No more elements. */
+    if (iterStack == NULL) {
+        return NULL;
+    }
+
+    /* Visit next node in-order. */
+    TreeNode *node = (TreeNode *)top(iterStack);
+    iterStack = pop(iterStack);
+
+    /* After visiting, push the left spine of the right subtree. */
+    for (TreeNode *n = node->right; n != NULL; n = n->left) {
+        iterStack = push(iterStack, n);
+    }
+
+    return node->data;
 }
 
 // Releases all memory resources (including data copies).