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;
}
};