110. Balanced Binary Tree

这道题重点是要明白平衡二叉树的定义。题目中给出的定义就是左右子树的高度不超过一。参考了以下链接：

https://blog.csdn.net/DERRANTCM/article/details/47414041

代码如下：

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11 
12     public boolean isBalanced(TreeNode root) {
13         
14         if(root == null){
15             return true;
16         }
17         
18         int left = depth(root.left);
19         int right = depth(root.right);
20         
21         if( Math.abs(left - right ) > 1){
22             return false;
23         }else{
24             return isBalanced(root.left) && isBalanced(root.right);
25         }
26     }
27     
28     private int depth(TreeNode n){
29         if( n == null){
30             return 0;
31         }
32         
33         if (n.left == null && n.right == null) {
34             return 1;
35         } else {
36             int left = depth(n.left);
37             int right = depth(n.right);
38             return 1 + (left > right ? left : right);
39         }
40     }
41     
42 
43 }

END