/**
 * 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) {
        //创建一个链表
        ListNode dummy = new ListNode(-1);
        //创建一个指针执行之前头节点
        ListNode cur = dummy;
        //当l1与l2不等于空  就循环
        while(l1 != null && l2 != null){
            //如果l1<l2
            if(l1.val < l2.val){
                //就把l1这个节点的接到cur上
                cur.next = l1;
                //向后更新cur与l1;
                cur = l1;
                l1 = l1.next;
            }else{
                cur.next = l2;
                cur = l2;
                l2 = l2.next;
            }
        }
        //上面循环完成后，要么l1为null,要么l2为空
        if(l1 != null) cur.next = l1;
        else cur.next  = l2;
        //返回头节点
        return dummy.next;
    }
}