兩個有序鏈表的合併

已知兩個非降序鏈表序列S1與S2，設計函數構造出S1與S2合併後的新的非降序鏈表S3。

一手編的第一個程式啊啊，好多小錯誤，調試了好久，不上手真的會忽視掉好多細節。。

import java.util.*;

class Node{ //結點類
 public int data;
 Node next = null;
 public Node(int data) {
  this.data = data;
 }
}

public class Solution {

 public static class  Link{ //鏈表類
  Node head = null;
  Node tem = null; 
  
  public void addnode(int d) {
   Node n = new Node(d);
   if(head == null)
    {head = n;
    tem = head;
    return;}
   tem.next = n;
   while(tem.next != null)//確保tem在鏈尾
    tem = tem.next;
    }
  
  public int finddata(Node d) {
   return d.data;
   }
  public Link merge(Link a, Link b) {
   a.tem = a.head;
   b.tem = b.head;
   Link c = new Link();
   
   while(a.tem != null && b.tem != null) {
    if(a.finddata(a.tem)<=b.finddata(b.tem)) {
     c.addnode(a.finddata(a.tem));
     a.tem = a.tem.next;
    }
    else{
     c.addnode(b.finddata(b.tem));
     b.tem = b.tem.next;
    }
   }
   while(a.tem != null) { 
    c.addnode(a.finddata(a.tem));
    a.tem = a.tem.next;
   }
   while(b.tem != null) {
    c.addnode(b.finddata(b.tem));
    b.tem = b.tem.next;
   }
   return c;
  }
  public void print(Link d) {
   d.tem = d.head;
   if(d.tem == null)
   {System.out.println("null");}
   else{
     while(d.tem.next != null) {
     System.out.print(d.tem.data +" ");
     d.tem = d.tem.next;
        }
     System.out.print(d.tem.data);
   }
  }
 }

 
 
 public static void main(String[] args) {
  Scanner in = new Scanner(System.in);

  Link s1 = new Link();
  Link s2 = new Link();
  
  int count;
  do {
  count = in.nextInt();
  if(count != -1)
   s1.addnode(count);
  }while(count != -1);
   
  do {
   count = in.nextInt();
   if(count != -1)
    s2.addnode(count);
   }while(count != -1);
  
  Link s3 = new Link().merge(s1, s2);
  
  new Link().print(s3);
   
  
  in.close();
   }
 }

1.最初寫finddata的輸入時只寫了tem，結果會發生NullPointerException 空指針異常；

2.在賦值給c的時候，最初寫的是c.tem.data = a.finddata.

兩個錯誤的本質都是對鏈表的理解還不深，只有創建結點才能賦值，每一個鏈表都有單獨的引用。

3.最終結果在大規模輸入時記憶體超限，確實merge部分有很多重覆代碼，需要優化。