Leetcode 101. Symmetric Tree(easy)

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.

/*
递归实现： 要考虑到左右两个子树去递归，所以重写函数
 
*/
/**
 * 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 helper(TreeNode* left, TreeNode* right){
        if (left == NULL && right == NULL){
            return true;
        }else if (left == NULL && right != NULL){
            return false;
        }else if (left != NULL && right == NULL){
            return false;
        }else if (left -> val == right -> val){
            return helper(left -> left, right -> right) && helper(left -> right, right -> left);
        }else{
            return false;
        }
    }
    bool isSymmetric(TreeNode* root) {
        if (root == NULL){
            return true;
        }
        return helper(root -> left, root -> right);
    }
};