Binary Tree Postorder Traversal

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

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

return [3,2,1].

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

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<int> postorderTraversal(TreeNode *root) {
13         
14         vector<int> vec;
15         getVal(vec, root);
16         return vec;
17     }
18     
19     void getVal(vector<int> &vec, TreeNode *root)
20     {
21         if(root != NULL)
22         {
23             getVal(vec, root->left);
24             getVal(vec, root->right);
25             vec.push_back(root->val);
26         }
27     }
28 };