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

解题思路：

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