LeetCode --- Balanced Binary Tree

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10  class Solution {
11 public:
12     int isBalanced_helper(TreeNode *root, bool &result) {
13         if (root == NULL) return 0;
14         if (!result) return 0;
15         int leftDepth = isBalanced_helper(root->left, result);
16         int rightDepth = isBalanced_helper(root->right, result);
17         if (abs(leftDepth - rightDepth) > 1) result = false;
18         // prevent overwrite of "result"
19         if (!result) return 0;
20         return max(leftDepth, rightDepth) + 1;
21     }
22     
23     bool isBalanced(TreeNode *root) {
24         bool result = true;
25         isBalanced_helper(root, result);
26         return result;
27     }
28 };