eetcode第21题合并有序链表

合并有序链表需要注意边界值：
1、链表为空的特殊值需要注意处理

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode l3 = new ListNode(0);
        l3.next=l1;
        ListNode prel1=l3;
        
        if(l1==null)return l2;
        if(l2==null)return l1;
        
        while(l1!=null&&l2!=null){
            if(l1.val<=l2.val){
                prel1=l1;
                l1=l1.next; 
            }else{
                ListNode temp= l2.next;
                l2.next = l1;
                prel1.next = l2;
                prel1=l2;
                l2=temp;
                temp=null;
            }
        }
        if(l1==null){
            prel1.next = l2;
        }
        return l3.next;
    }
}