Leetcode 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.

递归大法，代码如下：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool isSymmetric(TreeNode* root) {        if(root == NULL)            return true;        return symetric(root->left,root->right);    }        bool symetric(TreeNode* left,TreeNode* right)    {        if(left == NULL || right == NULL)            return left == right;        else if(left->val == right->val)        {            return symetric(left->left,right->right) && symetric(left->right,right->left);        }        else            return false;    }};