以下代码实现了二叉树的中序遍历。输入以下二叉树，中序遍历结果是4 2 5 1 3 6。

1//   12//  / \3//  2 34// / \ \5// 4 5  66 7struct TreeNode {8   int val;9   TreeNode* left;10   TreeNode* right;11   TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}12};13 14void inorderIterative(TreeNode* root) {15   stack<TreeNode*> st;16   TreeNode* curr = root;17 18   while (curr || !st.empty()) {19      while (curr) {20        st.push(curr);21        curr = curr->left;22      }23      curr = st.top(); st.pop();24      cout << curr->val << " ";25      curr = curr->right;26   }27}