方法1：自上而下递归

时间复杂度：$O(n^2)$

空间复杂度：$O(n)$

解题思路

https://leetcode.cn/problems/balanced-binary-tree/solution/ping-heng-er-cha-shu-by-leetcode-solution/

Java 代码

/**
 * Definition for a binary tree node.
 * public 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 {
    public boolean isBalanced(TreeNode root) {
        if (root == null) return true;
        if (Math.abs(height(root.left) - height(root.right)) > 1) {
            return false;
        } else {
            return isBalanced(root.left) && isBalanced(root.right);
        }
    }

    public int height(TreeNode node) {
        if (node == null) {
            return 0;
        } else {
            return Math.max(height(node.left), height(node.right)) + 1;
        }
    }
}