Balanced Binary Tree

来源：Leetcode

原帖：https://oj.leetcode.com/problems/balanced-binary-tree/

题目：
Given a binary tree, determine if it is height-balanced. For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

代码：

 /**  
  * 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) {  
     int height = 0;  
     return isBalancedHelper(root, height);  
   }  
     
   bool isBalancedHelper(TreeNode *root, int &height){  
     if (!root) return true;    
     int leftHeight = 0, rightHeight = 0;  
     if (!isBalancedHelper(root->left, leftHeight)) return false;  
     if (!isBalancedHelper(root->right, rightHeight)) return false;  
     if (abs(leftHeight - rightHeight) > 1) return false;  
     height = 1 + max(leftHeight, rightHeight);  
     return true;  
   }  
 };