101. Symmetric Tree

题目

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:

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

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。