# 面试题55 - II

输入一棵二叉树的根节点，判断该树是不是平衡二叉树。如果某二叉树中任意节点的左右子树的深度相差不超过1，那么它就是一棵平衡二叉树。

```
示例 1:

给定二叉树 [3,9,20,null,null,15,7]

    3
   / \
  9  20
    /  \
   15   7

返回 true 。

示例 2:

给定二叉树 [1,2,2,3,3,null,null,4,4]

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4

返回 false 。
```

## 限制：

* 1 <= 树的结点个数 <= 10000

## 注意：本题与主站 110 题相同：<https://leetcode-cn.com/problems/balanced-binary-tree/>

## Solutions

1. **recursion**

```cpp
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int depth(TreeNode * root) {
        if (!root) return 0;
        int lh = depth(root->left);
        int rh = depth(root->right);
        return (lh == -1 || rh == -1 || abs(lh - rh) > 1) 
                ? -1 : max(lh, rh) + 1;
    }
    bool isBalanced(TreeNode* root) {
        return depth(root) != -1;
    }
};
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://zhongquan789.gitbook.io/leetcode/lcof/mian-shi-ti-55-ii.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.