Balanced Binary Tree

 1 class Solution {
 2 public:
 3     bool isBalanced( TreeNode *root ) {
 4         return IsBalancedSub( root ) != -1;
 5     }
 6 private:
 7     int IsBalancedSub( TreeNode *node ) {
 8         if( !node ) { return 0; }
 9         int left = IsBalancedSub( node->left );
10         if( left == -1 ) { return -1; }
11         int right = IsBalancedSub( node->right );
12         if( right == -1 ) { return -1; }
13         if( abs( left-right ) > 1 ) { return -1; }
14         return max( left, right ) + 1;
15     }
16 };