[LeetCode]--101. Symmetric Tree

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.

一开始我就想到了递归算法，看图写程序嘛，简单可靠。

public boolean isSymmetric(TreeNode root) {
        if (root == null)
            return true;
        return testSymmetric(root.