leetcode

0 class Solution {

1 public:

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

3 if (!root) return {};

4

5 vector<int> res;

6 unordered_set<TreeNode *> s;

7 stack<TreeNode *> st;

8

9 while (root) {

10 st.push(root);

11 root = root->left;

12 }

13

14 while (!st.empty()) {

15 TreeNode *root = st.top();

16 st.pop();

17 if (!s.count(root)) {

18 s.insert(root);

19 st.push(root);

20 root = root->right;

21 while (root) {

22 st.push(root);

23 root = root->left;

24 }

25 } else {

26 res.push_back(root->val);

27 }

28 }

29

30 return res;

31 }

32 };