LintCode 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: The root of binary tree.
* @return: Preorder in vector which contains node values.
*/
vector<int> v;
void mid(TreeNode *root)
{
if(root==NULL)
return ;
v.push_back(root->val);
mid(root->left);
mid(root->right);
}
vector<int> preorderTraversal(TreeNode *root) {
// write your code here
mid(root);
return v;
}
};