Leetcode -- Binary Tree Preorder Traversal

 /**  
  * Definition for binary tree  
  * struct TreeNode {  
  *   int val;  
  *   TreeNode *left;  
  *   TreeNode *right;  
  *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
  * };  
  */  
 class Solution {  
 public:  
   vector<int> preorderTraversal(TreeNode *root) {  
     vector<int> result;  
     if(root == NULL) {  
       return result;  
     }  
     result.emplace_back(root->val);  
     if(root->left != NULL) {  
       auto ret = preorderTraversal(root->left);  
       for(int i : ret) {  
         result.emplace_back(i);  
       }  
     }  
      if(root->right != NULL) {  
       auto ret = preorderTraversal(root->right);  
       for(int i : ret) {  
         result.emplace_back(i);  
       }  
     }  
   }  
 };