leetcode 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.

求一个二叉树是否是平衡二叉树。平衡二叉树是每个节点的左子树和右子树的深度的绝对值之差不超过1的二叉树。

这道题可以借鉴前几天做过的求二叉树的深度的题，递归调用，每次记录左子树是否为平衡二叉树，右子树是否为平衡二叉树，左右子树是否平衡。 

如果不平衡，maxDepth返回－1

/**
 * 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) {
         return maxDepth(root) != -1;
    }
    
    private int maxDepth(TreeNode root) {
         if (root == null) {
             return 0;
         }
         
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        if (left == -1 || right == -1 || Math.abs(left - right) > 1){
            return -1;
        }else {
            return Math.max(left, right) + 1;
        }
        
    }
}