LinkedList 删除指定节点之前的节点

Question

我实现了 2 个方法，即 RemoveAfter 和 RemoveBefore，它们将在指示节点之前/之后删除节点。方法 Remove After 工作正常，但我不知道为什么 RemoveBefore 给我错误，因为 front 总是为空。

    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Question7
{
    public class LinkedListNode
    {
        public LinkedListNode next = null;
        public int data;
        public LinkedListNode(int d) { data = d; }
        public void AppendToTail(int d)
        {
            LinkedListNode end = new LinkedListNode(d);
            LinkedListNode n = this;
            while (n.next != null) { n = n.next; }
            n.next = end;
        }
    }

    class Program
    {
        #region DeleteAFter
        public static bool DeleteAfter(LinkedListNode n)
        {
            if (n == null || n.next == null)
            {
                return false; // Failure
            }

            LinkedListNode next = n.next;
            n.next = next.next;
            return true;
        }
        #endregion

        static LinkedListNode front;
        #region DeleteBefore
        public static bool DeleteBefore(LinkedListNode n)
        {
            LinkedListNode prev = null;
            LinkedListNode curr = front;
            prev = curr;

            while(curr.data != n.data)
            {
                prev = curr;
                curr = curr.next;
            }
            prev.data = curr.data;
            prev.next = curr.next;
            return true;
        }

        #endregion

        static void PrintList(LinkedListNode list)
        {
            while (list != null)
            {
                Console.Write(list.data + "->");
                list = list.next;
            }
            Console.WriteLine("null");
        }
        static void Main(String[] args)
        {
            LinkedListNode myList = new LinkedListNode(5);
            myList.AppendToTail(6);
            myList.AppendToTail(7);
            myList.AppendToTail(8);
            // Now the list is 5->6->7->8

            Console.Write("Before deletion: ");
            PrintList(myList); // 5->6->7->8->null 

            LinkedListNode deletedNode = myList;

            int val = 7;

            while (deletedNode.data != val)
            {
                deletedNode = deletedNode.next;
            }

            Console.Write("After deletion: ");
            if (DeleteBefore(deletedNode))
                PrintList(myList); 
            Console.Read();
        }
    }
}

Answer 1

因为您无法从一个节点访问其前一个节点，所以在DeleteBefore 上您必须从根目录开始再次遍历列表：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Question7
{
    public class LinkedListNode
    {
        public LinkedListNode next = null;
        public int data;
        public LinkedListNode(int d) { data = d; }
        public void AppendToTail(int d)
        {
            LinkedListNode end = new LinkedListNode(d);
            LinkedListNode n = this;
            while (n.next != null) { n = n.next; }
            n.next = end;
        }
    }

    class Program
    {
        #region DeleteAFter
        public static bool DeleteAfter(LinkedListNode n)
        {
            if (n == null || n.next == null)
            {
                return false; // Failure
            }

            LinkedListNode next = n.next;
            n.next = next.next;
            return true;
        }
        #endregion

        static LinkedListNode front;
        #region DeleteBefore
        public static bool DeleteBefore(LinkedListNode root, LinkedListNode n)
        {
            LinkedListNode prev = root;
            LinkedListNode curr = n;

            // we're iterating from the root, looking if the next node is the one we're looking for.
            while (prev.next != null && curr.data != prev.next.data)
            {
                prev = prev.next;
            }

            // if we found it
            if (prev.next != null && curr.data == prev.next.data)
            {
                // we're rewiring our list. we can also prev.next = curr.next;
                prev.next = prev.next.next;
                return true;
            }

            return false;
        }

        #endregion

        static void PrintList(LinkedListNode list)
        {
            while (list != null)
            {
                Console.Write(list.data + "->");
                list = list.next;
            }
            Console.WriteLine("null");
        }
        static void Main(String[] args)
        {
            LinkedListNode myList = new LinkedListNode(5);
            myList.AppendToTail(6);
            myList.AppendToTail(7);
            myList.AppendToTail(8);
            // Now the list is 5->6->7->8

            Console.Write("Before deletion: ");
            PrintList(myList); // 5->6->7->8->null 

            LinkedListNode deletedNode = myList;

            int val = 7;

            while (deletedNode.data != val)
            {
                deletedNode = deletedNode.next;
            }

            Console.Write("After deletion: ");
            if (DeleteBefore(myList, deletedNode))
                PrintList(myList);
            Console.Read();
        }
    }
}

Answer 2

您的front 变量始终为空。那么如果它始终为空，它有什么用？你正在与它进行比较，所以你得到null exception。 您可以在不使用 front 变量的情况下执行此操作。

     public static LinkedListNode DeleteBefore(LinkedListNode n,LinkedListNode currentList)
    {
        LinkedListNode prev = currentList;
        while (prev.data!=n.data)
        {
            prev = prev.next;
        }

        currentList = prev;

        return currentList;
    }


         LinkedListNode myList = new LinkedListNode(5);
        myList.AppendToTail(6);
        myList.AppendToTail(7);
        myList.AppendToTail(8);
        // Now the list is 5->6->7->8

        Console.Write("Before deletion: ");
        PrintList(myList); // 5->6->7->8->null 

        LinkedListNode deletedNode = myList;

        int val = 7;

        while (deletedNode.data != val)
        {
            deletedNode = deletedNode.next;
        }

        Console.Write("After deletion: ");
        PrintList(DeleteBefore(deletedNode,myList));

       }

Answer 3

前面变量中的值始终为空。稍后您将 curr 分配给 front 并尝试访问无效的 curr.data。您需要在 curr = 'this' 的地方实现 DeleteBefore 函数，它应该从 main 调用 myList.DeleteBefore(deletedNode)

        public static bool DeleteBefore(LinkedListNode n)
    {
        LinkedListNode prev = null;
        LinkedListNode curr = this;

        while(curr != null)
        {
            if(curr.next.data == n.data)
//if the next node has the same data as n,
// then curr node has to be deleted
                {
                     prev.next = curr.next;
                     return true;
                 }
            prev = curr;
            curr = curr.next;
        }
        return false;
    }

在main中，你调用这个函数如下：

myList.DeleteBefore(deletedNode);