【面试题十五】链表中倒数第k个结点

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:

#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;}// ====================测试代码====================// 测试要找的结点在链表中间void Test1(){    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);}// 测试要找的结点是链表的尾结点void Test2(){    printf("=====Test2 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: 5.\n");    ListNode* pNode = FindKthToTail(pNode1, 1);    PrintListNode(pNode);    DestroyList(pNode1);}// 测试要找的结点是链表的头结点void Test3(){    printf("=====Test3 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: 1.\n");    ListNode* pNode = FindKthToTail(pNode1, 5);    PrintListNode(pNode);    DestroyList(pNode1);}// 测试空链表void Test4(){    printf("=====Test4 starts:=====\n");    printf("expected result: NULL.\n");    ListNode* pNode = FindKthToTail(NULL, 100);    PrintListNode(pNode);}// 测试输入的第二个参数大于链表的结点总数void Test5(){    printf("=====Test5 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: NULL.\n");    ListNode* pNode = FindKthToTail(pNode1, 6);    PrintListNode(pNode);    DestroyList(pNode1);}// 测试输入的第二个参数为0void Test6(){    printf("=====Test6 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: NULL.\n");    ListNode* pNode = FindKthToTail(pNode1, 0);    PrintListNode(pNode);    DestroyList(pNode1);}int main(){Test1();    Test2();    Test3();    Test4();    Test5();    Test6();return 0;}

List.h:

#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:

#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:

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

运行结果：

=====Test1 starts:=====expected result: 4.The key in node is 4.=====Test2 starts:=====expected result: 5.The key in node is 5.=====Test3 starts:=====expected result: 1.The key in node is 1.=====Test4 starts:=====expected result: NULL.The node is NULL=====Test5 starts:=====expected result: NULL.The node is NULL=====Test6 starts:=====expected result: NULL.The node is NULL