/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode merge(ListNode l1, ListNode l2) {
        if(l1 == null && l2 == null){return null;}
        ListNode result = new ListNode(-1);
        ListNode temp = result;
        while(l1 != null && l2 != null){
            if(l1.val > l2.val){
                temp.next = l2;
                l2 = l2.next;
            }else{
                temp.next = l1;
                l1 = l1.next;
            }
            temp = temp.next;
        }
        // 还会有l1 或者 l2 未遍历完的情况
        while(l1 != null){
            temp.next = l1;
            l1 = l1.next;
            temp = temp.next;
        }
        while(l2 != null){
            temp.next = l2;
            l2 = l2.next;
            temp = temp.next;
        }
        return result.next;

    }
}