leetcode Merge Two Sorted Lists 2.9 难度系数2

Question:

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;
*         next = null;
*     }
* }
*/
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
ListNode p, q, r, head;
if (l1 == null && l2 == null)
return null;
if (l1 == null)
return l2;
if (l2 == null)
return l1;
p = l1;
q = l2;
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;
}
}
if(p==null){
r.next = q;
}else {
r.next = p;
}
return head;

}
}