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);      }};