diff --git a/lectures/21_trees_IV/postorder_iterative.cpp b/lectures/21_trees_IV/postorder_iterative.cpp
index 0c035e6..d17d836 100644
--- a/lectures/21_trees_IV/postorder_iterative.cpp
+++ b/lectures/21_trees_IV/postorder_iterative.cpp
@@ -9,7 +9,7 @@ public:
     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 };
 
-// pre order traverse a binary tree, iteratively.
+// post order traverse a binary tree, iteratively.
 void postorderTraversal(TreeNode* root) {
     if (root == nullptr) return;