leetCode---Symmetric Tree

一. 题目：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.
二. 思路分析
       题目大意：给定一颗二叉树，检查是否为它自己的镜像树。思路比较简单：直接借助递归，很容易写下如下代码：

class Solution {
public:
bool isSymmetric(TreeNode *root)
{
if (!root) return true;
return isSymmetric(root->left, root->right);
}
bool isSymmetric(TreeNode *lt, TreeNode *rt)
{
if (!lt && !rt) return true;
if (lt && !rt || !lt && rt || lt->val != rt->val) return false;
return isSymmetric(lt->left, rt->right) &&isSymmetric(lt->right, rt->left);
}
};