LeetCode 101 Symmetric Tree 判断一颗二叉树是否是镜像二叉树

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.

/**
 * 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==nullptr)
            return true;
        return helper(root->left,root->right);
    }
    bool helper(TreeNode* left,TreeNode* right)
    {
        if(!left&&!right)
            return true;
        else if(!left||!right)
            return false;
        else if(left->val!=right->val)
            return false;
        else
            return helper(left->left,right->right)&&helper(left->right,right->left);
    }
};