leetcode - Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
struct ListNode
{
	int val;
	ListNode *next;
	ListNode(int x) : val(x), next(NULL) {}
};
class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
		if(l1 == NULL && l2 == NULL) return NULL;
		ListNode *node = new ListNode(-1);
		ListNode *p = node;
		ListNode *cur = NULL;
		while (l1 != NULL || l2 != NULL)
		{
			if(l1 == NULL)
			{
				cur = l2;
				l2 = l2->next;
			}
			else if(l2 == NULL)
			{
				cur = l1;
				l1 = l1->next;
			}
			else
			{
				if(l1->val > l2->val)
				{
					cur = l2;
					l2 = l2->next;
				}
				else
				{
					cur = l1;
					l1 = l1->next;
				}
			}
			p->next = cur;
			p = p->next;
		}
		return node->next;
    }
};