leetcode 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 binary tree * 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<TreeNode *> tree;        tree.push(root);        TreeNode *q=NULL;        vector<int> a;        if(root==NULL) return a;        while(!tree.empty())        {            TreeNode *p=tree.top();            if((p->left==NULL&&p->right==NULL)||(q!=NULL&&(q==p->left||q==p->right)))            {                a.push_back(p->val);                q=p;                tree.pop();            }            else            {                if(p->right!=NULL)                {                    tree.push(p->right);                    q=p->right;                }                if(p->left!=NULL)                {                    tree.push(p->left);                    q=p->left;                }            }        }        return a;    }};