面试题15、输入一个链表，输出该链表中倒数第k个结点。

/*
brief: 输入一个链表，输出该链表中倒数第k个结点。
*/

#include "stdafx.h"
#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0
void 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 _tmain(int argc, _TCHAR *argv[])
{
Test1();
Test2();
Test3();
Test4();
Test5();
Test6();

return 0;
}

<pre name="code" class="cpp">

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

__declspec(dllexport) ListNode *CreateListNode(int value);
__declspec(dllexport) void ConnectListNodes(ListNode *pCurrent, ListNode *pNext);
__declspec(dllexport) void PrintListNode(ListNode *pNode);
__declspec(dllexport) void DestroyList(ListNode *pHead);

#include "List.h"
#include "stdafx.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);	// exit(1)表示异常退出.这个1是返回给操作系统的。
}
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 DestroyList(ListNode *pHead)
{
ListNode * pNode = pHead;
while(pNode != NULL)
{
pHead = pHead->m_pNext;
delete pNode;
pNode = pHead;
}
}