leetcode

0 class Solution {

1 public:

2 vector<int> preorderTraversal(TreeNode *root) {

3 if (!root) return {};

4

5 vector<int> res;

6 stack<TreeNode *> st;

7 st.push(root);

8 while (!st.empty()) {

9 TreeNode *root = st.top();

10 st.pop();

11 while (root) {

12 res.push_back(root->val);

13 if (root->right) st.push(root->right);

14 root = root->left;

15 }

16 }

17 return res;

18 }

19 };