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Java Solution for LeetCode algorithm problems, continually updating

There is a newer version: 1.24

101\. Symmetric Tree

Easy

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?