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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.

递归求解

代码如下：

public class Solution {
       public boolean isSymmetric(TreeNode root) {        return symmetricJudge(root, root);    }    public boolean symmetricJudge(TreeNode n1, TreeNode n2) {        if(n1 == null && n2 == null) return true;        if(n1 == null || n2 == null) return false;        return n1.val == n2.val && symmetricJudge(n1.left, n2.right) && symmetricJudge(n1.right, n2.left);    }}

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