leetcode 100 same tree

Given two binary trees, write a function to check if they are equal or not.
Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

/**
 * 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 isSameTree(TreeNode* p, TreeNode* q) {
        if(p == NULL && q == NULL) {
            return true;
        }
        else if(p == NULL || q == NULL || p->val != q->val) {
            return false;
        }
        else {
            return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
        }
    }
};