Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes' values.

For example:
Given binary tree [1,null,2,3],

   1
    \
     2
    /
   3

return [1,3,2].

 /**  
  * Definition for a binary tree node.  
  * struct TreeNode {  
  *   int val;  
  *   TreeNode *left;  
  *   TreeNode *right;  
  *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
  * };  
  */  
 class Solution {  
 public:  
   vector<int> inorderTraversal(TreeNode *root) {   
     vector<int> result;   
     inorderTra(root, result);   
     return result;  
   }   
   void inorderTra(TreeNode* node, vector<int> &result) {   
     if(node == NULL) {      
       return;  
     }   
     inorderTra(node->left, result);   
     result.push_back(node->val);     
     inorderTra(node->right, result);   
   }  
 };