LeetCode--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?

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