leetcode Symmetric Tree

SymmetricTree.cpp

/*
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.
*/

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