leetcode 110

110. Balanced Binary Tree

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.

判断二叉树是否为平衡二叉树。

递归解法：
（1）如果二叉树为空，返回真
（2）如果二叉树不为空，如果左子树和右子树都是AVL树并且左子树和右子树高度相差不大于1，返回真，其他返回假

代码如下：

 1 /**
 2  * Definition for a binary tree node.
 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     bool isBalanced(TreeNode* root) {
13         int height;
14         return isAvl(root, height);
15     }
16     bool isAvl(TreeNode* pRoot, int & height)
17     {
18         if(pRoot == NULL)
19         {
20             height = 0;
21             return true;
22         }
23         int heightLeft;
24         bool resultLeft = isAvl(pRoot->left, heightLeft);
25         int heightRight;
26         bool resultRight = isAvl(pRoot->right, heightRight);
27         if(resultLeft && resultRight && abs(heightLeft - heightRight) <= 1)
28         {
29             height = max(heightLeft, heightRight) + 1;
30             return true;
31         }
32         else
33         {
34             height = max(heightLeft, heightRight) + 1;
35             return false;
36         }
37     }
38 };