# 28.对称的二叉树

## 题目

请实现一个函数，用来判断一棵二叉树是不是对称的。如果一棵二叉树和它的镜像一样，那么它是对称的。

例如，二叉树 \[1,2,2,3,4,4,3] 是对称的。

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

但是下面这个 \[1,2,2,null,3,null,3] 则不是镜像对称的:

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

示例 1：

输入：root = \[1,2,2,3,4,4,3] 输出：true 示例 2：

输入：root = \[1,2,2,null,3,null,3] 输出：false

限制：

0 <= 节点个数 <= 1000

注意：本题与主站 101 题相同：<https://leetcode-cn.com/problems/symmetric-tree/>

来源：力扣（LeetCode） 链接：<https://leetcode-cn.com/problems/dui-cheng-de-er-cha-shu-lcof> 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

## 题解

递归

```swift
func isSymmetric(_ root: TreeNode?) -> Bool {
    func compare(tree1: TreeNode?, tree2: TreeNode?) -> Bool {
        if tree1 == nil && tree2 == nil {
            return true
        }
        else if tree1 == nil || tree2 == nil {
            return false
        }
        else if tree1?.val != tree2?.val {
            return false
        }
        return compare(tree1: tree1?.left, tree2: tree2?.right) && compare(tree1: tree1?.right, tree2: tree2?.left)
    }

    return compare(tree1: root?.left, tree2: root?.right)
}
```


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