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) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode *l = NULL, *head = NULL, *p = l1, *q = l2;
if(l1 == NULL)
return l2;
if(l2 == NULL)
return l1;

while(p && q)
{
if(head == NULL)
{
if(p->val < q->val)
{
head = l = p;
p = p->next;
l->next = NULL;
}
else
{
head = l = q;
q = q->next;
l->next = NULL;
}
}
else
{
if(p->val < q->val)
{
l->next = p;
l = p;
p = p->next;
l->next = NULL;
}
else
{
l->next = q;
l = q;
q = q->next;
l->next = NULL;
}
}

}
if(p)
l->next = p;
if(q)
l->next = q;
return head;
}
};