leetcode-110. Balanced Binary Tree

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;
    }

}