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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        vector<int> ret;
        if(root==NULL)return ret;
        stack<TreeNode*> st;
        st.push(root);
        TreeNode* ptr;
        while(st.size()!=0){
            ptr=st.top();
            if(ptr->left==NULL&&ptr->right==NULL){
                ret.push_back(ptr->val);
                st.pop();
            }
            if(ptr->right!=NULL)st.push(ptr->right);
            if(ptr->left!=NULL)st.push(ptr->left);
            ptr->left=NULL;
            ptr->right=NULL;
        }
        return ret;
    }
};