145

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;
        stack<TreeNode*> s;

        if(root) s.push(root);
        while(!s.empty()){
            TreeNode *tmp = s.top();
            s.pop();
            ans.push_back(tmp->val);
            if(tmp->left) s.push(tmp->left);
            if(tmp->right) s.push(tmp->right);
        }
        reverse(ans.begin(), ans.end());
        return ans;
    }
};