【面试题015】链表中倒数第k个结点

【面试题015】链表中倒数第k个结点

可以用两个指针，当第一个指针指向了第k个时候，第二个指针让他指向链表的第一个元素，然后这两个指针同时向后面移动，

当第一个指针移动到末尾的时候，第二个指针指向的就是倒数第K个结点；两个指针的间距保持为k-1；

当我们遍历列表的时候发现用一个指针是解决不了问题的，我们可以尝试用两个指针来解决问题，

一个指针走的比另外一个指针走得快一点，或者先让其中的一个指针走了若干步，然后再让第二个指针来走；

kth.cpp:

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#include <iostream>
#include <cstdio>
#include "List.h"

ListNode *FindKthToTail(ListNode *pListHead, unsigned int k)
{
    if(pListHead == NULL || k == 0)
    {
        return NULL;
    }

ListNode *pAhead = pListHead;
ListNode *pBehind = NULL;

for(unsigned int i = 0; i < k - 1; ++i)
    {
        if(pAhead->m_pNext != NULL)
        {
            pAhead = pAhead->m_pNext;
        }
        else
        {
            return NULL;
        }
    }

pBehind = pListHead;
    while(pAhead->m_pNext != NULL)
    {
        pAhead = pAhead->m_pNext;
        pBehind = pBehind->m_pNext;
    }
    return pBehind;
}

// 测试要找的结点在链表中间
int main()
{
    printf("=====Test1 starts:=====\n");
    ListNode *pNode1 = CreateListNode(1);
    ListNode *pNode2 = CreateListNode(2);
    ListNode *pNode3 = CreateListNode(3);
    ListNode *pNode4 = CreateListNode(4);
    ListNode *pNode5 = CreateListNode(5);

ConnectListNodes(pNode1, pNode2);
    ConnectListNodes(pNode2, pNode3);
    ConnectListNodes(pNode3, pNode4);
    ConnectListNodes(pNode4, pNode5);

printf("expected result: 4.\n");
ListNode *pNode = FindKthToTail(pNode1, 2);
PrintListNode(pNode);

DestroyList(pNode1);
return 0;
}

运行结果：

=====Test1 starts:=====

expected result: 4.

The key in node is 4.

请按任意键继续. . .

List.h:

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#ifndef _LIST_H_
#define _LIST_H_

struct ListNode
{
int m_nValue;
ListNode *m_pNext;
};

ListNode *CreateListNode(int value);
void ConnectListNodes(ListNode *pCurrent, ListNode *pNext);
void PrintListNode(ListNode *pNode);
void PrintList(ListNode *pHead);
void DestroyList(ListNode *pHead);
void AddToTail(ListNode **pHead, int value);
void RemoveNode(ListNode **pHead, int value);

#endif //_LIST_H_

List.cpp:

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#include "list.h"
#include <stdio.h>
#include <stdlib.h>

ListNode *CreateListNode(int value)
{
    ListNode *pNode = new ListNode();
    pNode->m_nValue = value;
    pNode->m_pNext = NULL;

return pNode;
}

void ConnectListNodes(ListNode *pCurrent, ListNode *pNext)
{
    if(pCurrent == NULL)
    {
        printf("Error to connect two nodes.\n");
        exit(1);
    }

pCurrent->m_pNext = pNext;
}

void PrintListNode(ListNode *pNode)
{
    if(pNode == NULL)
    {
        printf("The node is NULL\n");
    }
    else
    {
        printf("The key in node is %d.\n", pNode->m_nValue);
    }
}

void PrintList(ListNode *pHead)
{
printf("PrintList starts.\n");

ListNode *pNode = pHead;
    while(pNode != NULL)
    {
        printf("%d\t", pNode->m_nValue);
        pNode = pNode->m_pNext;
    }

printf("\nPrintList ends.\n");
}

void DestroyList(ListNode *pHead)
{
    ListNode *pNode = pHead;
    while(pNode != NULL)
    {
        pHead = pHead->m_pNext;
        delete pNode;
        pNode = pHead;
    }
}

void AddToTail(ListNode **pHead, int value)
{
    ListNode *pNew = new ListNode();
    pNew->m_nValue = value;
    pNew->m_pNext = NULL;

if(*pHead == NULL)
    {
        *pHead = pNew;
    }
    else
    {
        ListNode *pNode = *pHead;
        while(pNode->m_pNext != NULL)
            pNode = pNode->m_pNext;

pNode->m_pNext = pNew;
}
}

void RemoveNode(ListNode **pHead, int value)
{
if(pHead == NULL || *pHead == NULL)
return;

ListNode *pToBeDeleted = NULL;
    if((*pHead)->m_nValue == value)
    {
        pToBeDeleted = *pHead;
        *pHead = (*pHead)->m_pNext;
    }
    else
    {
        ListNode *pNode = *pHead;
        while(pNode->m_pNext != NULL && pNode->m_pNext->m_nValue != value)
            pNode = pNode->m_pNext;

if(pNode->m_pNext != NULL && pNode->m_pNext->m_nValue == value)
        {
            pToBeDeleted = pNode->m_pNext;
            pNode->m_pNext = pNode->m_pNext->m_pNext;
        }
    }

if(pToBeDeleted != NULL)
    {
        delete pToBeDeleted;
        pToBeDeleted = NULL;
    }
}

Makefile:

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.PHONY:clean
CPP=g++
CFLAGS=-Wall -g
BIN=test
OBJS=Kth.o List.o
LIBS=
$(BIN):$(OBJS)
    $(CPP) $(CFLAGS) $^ -o $@ $(LIBS)
%.o:%.cpp
    $(CPP) $(CFLAGS) -c $< -o $@
clean:
    rm -f *.o $(BIN)

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【面试题015】链表中倒数第k个结点

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