Leet Code 21 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.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
// Note: The Solution object is instantiated only once and is reused by each test case.
ListNode p,q,r,head;
if(l1 == null){
return l2;
}
if(l2 == null){
return l1;
}
p = l1;
q = l2;
head = null;
if(p.val <= q.val){
head = p;
p = p.next;
}
else{
head = q;
q = q.next;
}
r = head;
while(p != null && q != null){
if(p.val <= q.val){
r.next = p;
r = p;
p = p.next;
}
else{
r.next = q;
r = q;
q = q.next;

}
}

while(q != null){
r.next = q;
r = q;
q = q.next;
}
while(p != null)
{
r.next = p;
r = p;
p = p.next;

}
return head;
}
}