二叉树的前序遍历

/**
* 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: A Tree
　　* @return: Preorder in ArrayList which contains node values.
　　*/
　　vector <int> v;
　　vector<int> preorderTraversal(TreeNode * root) {
　　if (root == NULL) {
　　　　return v;
　　}
　　v.push_back(root->val);
　　preorderTraversal(root->left);
　　preorderTraversal(root->right);
　　return v;
　　}
};