1. 题目

2. 思路

(1) 递归

• 自底向上递归计算左右子树的高度差，设置全局变量记录二叉树是否平衡，若二叉树已经不平衡，则直接返回即可。

3. 代码

public class Test {
    
    
    public static void main(String[] args) {
    
    
    }
}

class TreeNode {
    
    
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    
    
    }

    TreeNode(int val) {
    
    
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
    
    
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

class Solution {
    
    
    private boolean res = true;

    public boolean isBalanced(TreeNode root) {
    
    
        getDepth(root);
        return res;
    }

    private int getDepth(TreeNode root) {
    
    
        if (!res) {
    
    
            return 0;
        }
        if (root == null) {
    
    
            return 0;
        }
        int left = getDepth(root.left);
        int right = getDepth(root.right);
        if (Math.abs(left - right) > 1) {
    
    
            res = false;
        }
        return Math.max(left, right) + 1;
    }
}