LeetCode OJ Binary Tree Preorder Traversal

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

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

   1    \     2    /   3

return [1,2,3].

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

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