leetcode 110. Balanced Binary Tree

 1 bool isBalanced(TreeNode *root) 
 2     {
 3         int depth;
 4         
 5         return balanceCheck(root, depth);
 6     }
 7     
 8     bool balanceCheck(TreeNode *root, int &root_depth)
 9     {
10         int left_depth, right_depth;
11         if (root == NULL)
12         {
13             root_depth = 0;
14             return true;
15         }
16         
17         if (balanceCheck(root->left, left_depth) && balanceCheck(root->right, right_depth))
18         {
19             if (abs(left_depth - right_depth) <= 1)
20             {
21                 root_depth = max(left_depth, right_depth) + 1;
22                 return true;
23             }
24         }
25             
26         return false;
27     }