21.Merge Two Sorted Lists（链表）

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) {
    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;
}
}