LeetCode OJ 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?

class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        ans.clear();
        if (root != NULL) gogogo(root);
        return ans;
    }
    void gogogo(TreeNode *nowRoot) {
        if (nowRoot->left != NULL) gogogo(nowRoot->left);
        if (nowRoot->right != NULL) gogogo(nowRoot->right);
        ans.push_back(nowRoot->val);
    }
    vector<int> ans;
};