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);
    }
};

原文：http://blog.csdn.net/shaya118/article/details/42738171