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 / \ 22 / \ / \ 3443

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

1 / \ 22 \ \ 33

Note:

Bonus points if you could solve it both recursively and iteratively.

这一题要求判断 2 颗树是否是左右对称的。

• 这道题是几道题的综合题。将根节点的左字数反转二叉树，然后再和根节点的右节点进行比较，是否完全相等。
• 反转二叉树是第 226 题。判断 2 颗树是否完全相等是第 100 题。