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;

	}
}