[LeetCode] Binary Tree Postorder Traversal

Question:

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?

Solution (C++):

 /**
 * 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> result;
        if (root == NULL) return result;
        stack<TreeNode*> com;
        com.push(root);
        TreeNode *temp = NULL;
        
        while(!com.empty())
        {
            temp = com.top();
            if(temp -> left == NULL && temp -> right == NULL)
            {
                result.push_back(temp -> val);
                com.pop();
            }
            else
            {
                if(temp -> right)
                {
                    com.push(temp -> right);
                    temp -> right = NULL;
                }
                if(temp -> left)
                {
                    com.push(temp -> left);
                    temp -> left = NULL;
                }

            }
        }
        return result;
        
    }
};