Create Symmetric Tree

还是利用递归的思想来做。注意递归的条件就好。必须要左右子树都不为空同时值相同时才可以递归。。。同时注意递归的截止。

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3
But the following is not:
    1
   / \
  2   2
   \   \
   3    3