101 Symmetric Tree

链接：https://leetcode.com/problems/symmetric-tree/
问题描述：
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

Note:
Bonus points if you could solve it both recursively and iteratively.

confused what “{1,#,2,3}” means? > read more on how binary tree is serialized on OJ.

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检查一棵树是否是对称的，这个问题可以用递归做。

class Solution {
public:

    bool isSymmetric(TreeNode* root) {
         if(root==NULL)return true;
         return mSymmetric(root->left,root->right);
    }

    bool mSymmetric(TreeNode* n1,TreeNode* n2)
    {
        if(n1==NULL&&n2==NULL)return true;
        else if(n1==NULL||n2==NULL) return false;
        else if(n1->val!=n2->val)return false;
        else return mSymmetric(n1->left,n2->right)&&mSymmetric(n1->right,n2->left);
    }
};