lintcode 二叉树前序遍历

二叉树的前序遍历 

给出一棵二叉树，返回其节点值的前序遍历。

您在真实的面试中是否遇到过这个题？

Yes

样例

给出一棵二叉树 {1,#,2,3},

   1
    \
     2
    /
   3

返回 [1,2,3].

 /**
  * 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;
     }
 };