145.Binary Tree Postorder Traversal

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        if (!root) return {};
        vector<int> res;
        stack<TreeNode*> s{{root}};
        TreeNode *head = root;
        while (!s.empty()) {
            TreeNode *t = s.top();
            if ((!t->left && !t->right) || t->left == head || t->right == head) {
                res.push_back(t->val);
                s.pop();
                head = t;
            } else {
                if (t->right) s.push(t->right);
                if (t->left) s.push(t->left);
            }
        }
        return res;
    }
};

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        if (!root) return {};
        vector<int> res;
        stack<TreeNode*> s{{root}};
        while (!s.empty()) {
            TreeNode *t = s.top(); s.pop();
            res.insert(res.begin(), t->val);
            if (t->left) s.push(t->left);
            if (t->right) s.push(t->right);
        }
        return res;
    }
};