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# 0101. Symmetric Tree

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

## Description

Given the `root` of a binary tree, *check whether it is a mirror of itself* (i.e., symmetric around its center).

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/02/19/symtree1.jpg)

```
**Input:** root = [1,2,2,3,4,4,3]
**Output:** true
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2021/02/19/symtree2.jpg)

```
**Input:** root = [1,2,2,null,3,null,3]
**Output:** false
```

**Constraints:**

* The number of nodes in the tree is in the range `[1, 1000]`.
* `-100 <= Node.val <= 100`

**Follow up:** Could you solve it both recursively and iteratively?

## ac

```java
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        return helper(root, root);
    }
    private boolean helper(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null) return true;
        if (t1 == null || t2 == null) return false;
        if (t1.val != t2.val) return false;
        return helper(t1.left, t2.right) && helper(t1.right, t2.left);
    } 
}
```