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) {
        
        vector<int> vec;
        getVal(vec, root);
        return vec;
    }
    
    void getVal(vector<int> &vec, TreeNode *root)
    {
        if(root != NULL)
        {
            getVal(vec, root->left);
            getVal(vec, root->right);
            vec.push_back(root->val);
        }
    }
};