/**

 * 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) {

    if(l1==null)

        return l2;

    if(l2==null)

        return l1;

    ListNode headNode=new ListNode(0);  //注意这里要先初始化

    ListNode head=headNode;

    head.next=null;

    ListNode r=head,p=l1,q=l2;

    while(p!=null&&q!=null){

        if(p.val<=q.val){

            r.next=p;            //想想为什么要r.next.    r不行是为什么

            r=r.next;

            p=p.next;

        }

        else{

            r.next=q;

            r=r.next;

            q=q.next;

        }

    }

    if(p!=null)

        r.next=p;

    else

        r.next=q;

    return head.next;

}

}