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);
     }
 };