145. Binary Tree Postorder Traversal

O(n) time O(h) space

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        stack<pair<TreeNode *, bool>> s;
        s.emplace(root, false);
        vector<int> res;
        while (!s.empty()) {
            auto [x, v] = s.top(); s.pop();
            if (!x) continue;
            if (v) {
                res.push_back(x->val);
            } else {
                s.emplace(x, true);
                s.emplace(x->right, false);
                s.emplace(x->left, false);
            }
        }
        return res;
    }
};