java - 如何删除链表中所有出现的项目？

Question

我是链表的初学者，这是我第一次尝试编写一个方法调用 removeall(N)，它将删除列表中某个元素的所有出现。我遍历列表，如果我看到一个等于 n 的元素，我会删除链接，但是我收到一条错误消息

 Exception in thread "main" 22 at LinkList.removeAll(LinkList.java:34)
java.lang.NullPointerException 

有人可以解释一下我做错了什么吗？

链接.java

class Link
   {
   public long iData;              // data item
   public Link next;              // next link in list
// -------------------------------------------------------------
   public Link(long id) // constructor
      {
      iData = id;                 // initialize data
                       // ('next' is automatically
      }                           //  set to null)
// -------------------------------------------------------------
   public void displayLink()      // display ourself
      {
      System.out.print("{" + iData  + "} ");
      }
   }  // end class Link

链表.java

class LinkList
{
    private Link first;            // ref to first item on list
    // -------------------------------------------------------------
    public LinkList()              // constructor
    { 
        first = null; }           // no items on list yet
    // -------------------------------------------------------------
    public boolean isEmpty()       // true if list is empty
    { 
        return (first==null); }
    // -------------------------------------------------------------
    public void insertFirst(long dd) // insert at start of list
    {                           // make new link
        Link newLink = new Link(dd);
        newLink.next = first;       // newLink --> old first
        first = newLink;            // first --> newLink
    }
    // -------------------------------------------------------------
    //public long deleteFirst()      // delete first item
    public Link deleteFirst()      // delete first item
    {                           // (assumes list not empty)
        Link temp = first;          // save reference to link
        first = first.next;         // delete it: first-->old next
        return temp;          // return deleted link
    }

    public void removeAll(int n) {

          Link current = first;       // start at beginning of list
          while(current != null)  {
              current = current.next; 

              if(current.iData==n) {
                  current.next=current.next.next;



              }




          }

    }
    // -------------------------------------------------------------
    public void displayList()
    {
        System.out.print("List (first-->last): ");
        Link current = first;       // start at beginning of list
        while(current != null)      // until end of list,
        {
             System.out.print("List (first-->last): "+ current.iData);    // print data
            current = current.next;  // move to next link
        }
        System.out.println("");
    }
    // -------------------------------------------------------------
}  // end class LinkList

标签： java

Answer 1

以下是您的 removeAll 实施中的一些问题：

1. if(current.iData==n)检查当前列表项的值，但 current.next=current.next.next;删除下一项。
2. 您不处理列表开头的匹配项。

尝试以下 removeAll 方法的实现：

        public void removeAll(int n) {
            // handling elements from the beginning
            while(first != null && first.iData == n){ 
                first = first.next;
            }

            // Removing matched elements from the rest of the list
            Link current = first;
            while (current != null && current.next != null ) {
                if (current.next.iData == n) {
                    current.next = current.next.next;
                } else {
                    current = current.next;
                }
            }
        }