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;
}
};