> For the complete documentation index, see [llms.txt](https://senliuy.gitbook.io/leetcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://senliuy.gitbook.io/leetcode/chapter1/15-shu/q101-symmetric-tree.md).

# Q101. Symmetric Tree

直达：<https://leetcode.com/problems/symmetric-tree/description/>

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree`[1,2,2,3,4,4,3]`is symmetric:

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

But the following`[1,2,2,null,3,null,3]`is not:

```
    1
   / \
  2   2
   \   \
   3    3
```

**Note:**\
Bonus points if you could solve it both recursively and iteratively.

## 分析

两棵树是对称的充要条件是第一棵树的左子树和第二棵树的右子树互相对称，并且第一棵树的右子树与第二棵树的左子树互相对称。

所以用一个判断两个数是否对称做辅助函数

## C++代码

```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:
    bool isSymmetric(TreeNode* root) {
        if(root==NULL) return true;
        return symmetricTrees(root->left, root->right);
    }
private:
    bool symmetricTrees(TreeNode* left, TreeNode* right){
        if(left==NULL && right==NULL) return true;
        if(left==NULL ^ right==NULL) return false;
        if(left->val != right->val) return false;
        if(!symmetricTrees(left->left, right->right)) return false;
        if(!symmetricTrees(left->right, right->left)) return false;
        return true;
    }
};
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## 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, and the optional `goal` query parameter:

```
GET https://senliuy.gitbook.io/leetcode/chapter1/15-shu/q101-symmetric-tree.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

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.