原题链接在这里：https://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 left and right subtrees of every node differ in height by no more than 1.
Example 1:
Given the following tree [3,9,20,null,null,15,7]:
3
/
9 20
/
15 7
Return true.
Example 2:
Given the following tree [1,2,2,3,3,null,null,4,4]:
1
/
2 2
/
3 3
/
4 4
Return false.
题解：
Get 最大深度.如果左右最大深度相差大于一，则返回-1. 若是已有左或者右的返回值为-1, 即返回-1.
最后看返回到root时是否为一个负数，若是负数则不是balanced, 若是正数，则返回了最大深度，是balanced.
Time Complexity: O(n).
Space: O(log n), n is number of nodes in the tree.
AC Java:
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 public class Solution { 11 public boolean isBalanced(TreeNode root) { 12 return maxDepth(root) >= 0; 13 } 14 15 private int maxDepth(TreeNode root){ 16 if(root == null){ 17 return 0; 18 } 19 int leftDepth = maxDepth(root.left); 20 int rightDepth = maxDepth(root.right); 21 if(leftDepth == -1 || rightDepth == -1 || Math.abs(leftDepth - rightDepth) > 1){ 22 return -1; 23 } 24 return Math.max(leftDepth, rightDepth) + 1; 25 } 26 }