Leetcode -- Balanced Binary Tree

 /**  
  * Definition for binary tree  
  * struct TreeNode {  
  *   int val;  
  *   TreeNode *left;  
  *   TreeNode *right;  
  *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
  * };  
  */  
 class Solution {  
 public:  
   bool isBalanced(TreeNode *root) {  
     if(root == NULL) {  
       return true;  
     }  
     int height = 0;  
     return isBalancedHelper(root, height);  
   }  
   bool isBalancedHelper(TreeNode * root, int& height) {  
     if(root == NULL) {  
       return 0;  
     }  
     int left = 0;  
     if(root->left != NULL) {  
       if(!isBalancedHelper(root->left, left)) {  
        return false;    
       }  
     }  
     int right = 0;  
     if(root->right != NULL) {  
       if(!isBalancedHelper(root->right, right)) {  
         return false;  
       }  
     }  
     if(std::abs(left-right) > 1) {  
       return false;  
     }  
     height = std::max(left, right) + 1;  
     return true;  
   }  
 };