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 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.

　递归：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool check(TreeNode *A, TreeNode * B)
    {
        if(A == NULL && B == NULL) return true;
        if(A == NULL || B == NULL) return false;
        if(A->val != B->val) return false;
  
        return check(A->left, B->right) && check(A->right , B->left) ;
    }
    bool isSymmetric(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return check(root, root) ;
    }
};