leetcode/Problems/0145.cpp at master · DimitrijeDobrota/leetcode · GitHub

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class Solution {
  public:
    vector<int> postorderTraversal(TreeNode *root) {
        if (!root) return {};

        vector<int> res;
        unordered_set<TreeNode *> s;
        stack<TreeNode *> st;

        while (root) {
            st.push(root);
            root = root->left;
        }

        while (!st.empty()) {
            TreeNode *root = st.top();
            st.pop();
            if (!s.count(root)) {
                s.insert(root);
                st.push(root);
                root = root->right;
                while (root) {
                    st.push(root);
                    root = root->left;
                }
            } else {
                res.push_back(root->val);
            }
        }

        return res;
    }
};