leetcode-110. 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.

这个算法，比较简单，但是其实仔细看到话其实重复计算了。不过我没想到有什么算法可以不用重复计算的。讨论里好像也没有。这里先这样吧。

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isBalanced(TreeNode root) {        if (root == null) return true;        return DFS(root)!=-1;    }    private int DFS(TreeNode root){        if(root == null) return 0;        int lh = DFS(root.left);        if(lh == -1) return -1;        int rh = DFS(root.right);        if(rh == -1) return -1;        if( lh > rh ? (lh - rh)>1 : (rh - lh)>1 ) return -1;        return Math.max(lh,rh) + 1;    }}