java-尾插法和头插法创建链表

import java.util.*;
class  ListNode{
    int val;
    ListNode next;

    public ListNode() {
    }

    public ListNode(int val) {
        this.val = val;
    }

    public ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}
public class Main {
    //合并有序链表
    public static ListNode merge(ListNode n1,ListNode n2){
        if(n1==null)
            return n2;
        else  if(n2==null){
            return  n1;
        }
        else if(n1.val