Lintcode 66 二叉树的前序遍历

描述：

给出一棵二叉树，返回其节点值的前序遍历。

样例：

给出一棵二叉树 {1,#,2,3},
 

返回 [1,2,3].

挑战：

你能使用非递归实现么？

代码：

/** * Definition of TreeNode: * class TreeNode { * public: *     int val; *     TreeNode *left, *right; *     TreeNode(int val) { *         this->val = val; *         this->left = this->right = NULL; *     } * } */class Solution {public:    /**     * @param root: The root of binary tree.     * @return: Preorder in vector which contains node values.     */    vector<int> v;        vector<int> preorderTraversal(TreeNode *root) {        // write your code here                if(root != NULL)        {            v.push_back(root->val);            preorderTraversal(root->left);            preorderTraversal(root->right);        }        return v;    }};