Leetcode70: Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1    \     2    /   3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

/** * 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:    void post(TreeNode* Node, vector<int>& res)    {        if(Node->left)            post(Node->left, res);        if(Node->right)            post(Node->right, res);        res.push_back(Node->val);    }    vector<int> postorderTraversal(TreeNode* root) {        vector<int> res;        if(!root)            return res;        post(root, res);        return res;    }};