Update binary-tree-preorder-traversal.cpp

Author: kamyu104

C++/binary-tree-preorder-traversal.cpp

@@ -49,8 +49,8 @@ class Solution2 {
         vector<int> res;
         stack<pair<TreeNode *, bool>> s;
         s.emplace(root, false);
-        bool visited;
         while (!s.empty()) {
+            bool visited;
             tie(root, visited) = s.top();
             s.pop();
             if (root == nullptr) {