/**

  * Definition of TreeNode:

  * class TreeNode {

  * public:

  *     int val;

  *     TreeNode *left, *right;

  *     TreeNode(int val) {

  *         this->val = val;

  *         this->left = this->right = NULL;

  *     }

  * }

  */

 //递归

 class Solution {

 public:

     /*

      * @param root: A Tree

      * @return: Preorder in ArrayList which contains node values.

      */

      void inorder (TreeNode *root, vector<int> &result) {

          result.push_back(root->val);

          if (root->left != NULL) {

              inorder(root->left, result);

          }

          if (root->right != NULL) {

              inorder(root->right, result);

          }

      }

     vector<int> preorderTraversal(TreeNode * root) {

         // write your code here

         vector<int> result;

         if (root == NULL) {

             return result;

         }

         inorder(root, result);

         return result;

     }

 };