输入一个单向链表，输出该链表中倒数第K个结点

#include <iostream>
using namespace std;
struct LinkNode
{
public:
LinkNode(int value = 0) : nValue(value){ pNext = NULL; }
~LinkNode() { pNext = NULL; }
//private:
friend class LinkList;
int nValue;
LinkNode *pNext;
};

//带头结点的链表
class LinkList
{
public:
LinkList();
~LinkList();
void Insert(int nData); //插入元素
void Delete(int nData); //删除元素
void Sort();    //排序
bool IsEmpty(); //判断是否为空
void Reverse(); //反转
void Destroy(); //销毁整个链表
int Length();   //链表长度
LinkNode* Find(int nData);  //查找元素
bool IsLoop();  //判断是否为环
void Print();   //打印链表
LinkNode *FindByIndex(int n);   //返回链表中第i个元素
LinkNode *FindLastNum(int n);   //使用两个指针 高效操作
private:
LinkNode *pHead;
};

LinkList::LinkList()
{
pHead = new LinkNode();
}
LinkList::~LinkList()
{
Destroy();
}
//从大到小插入
void LinkList::Insert(int nData)
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return;
}
LinkNode *pCur = pHead;
while (pCur->pNext != NULL)
{
if (pCur->pNext->nValue < nData)
{
break;
}
pCur = pCur->pNext;
}
LinkNode *pTmp = new LinkNode(nData);
pTmp->pNext = pCur->pNext;
pCur->pNext = pTmp;
}

void LinkList::Delete(int nData)
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return;
}
if (pHead->pNext == NULL)
{
cout << "链表为空!" << endl;
return;
}

LinkNode *pCur = pHead;
while (pCur->pNext)
{
if (pCur->pNext->nValue == nData)
{
LinkNode *pDel = pCur->pNext;
pCur->pNext = pCur->pNext->pNext;

pDel->pNext = NULL;
delete (pDel);
}
else
{
pCur = pCur->pNext;
}
}
}

void LinkList::Sort()
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return;
}
if (pHead->pNext == NULL)
{
cout << "链表为空!" << endl;
}

LinkNode *pCur = pHead->pNext;
LinkNode *pPre = pHead;

LinkNode *pNewHead = new LinkNode();
while (pCur)
{
LinkNode *pTmp = pCur;
pCur = pCur->pNext;

//将pTmp结点插入到pNewHead指向的新链表中
LinkNode *p = pNewHead;
while (p->pNext)
{
if (p->pNext->nValue > pTmp->nValue)
{
break;
}
p = p->pNext;
}
pTmp->pNext = p->pNext;
p->pNext = pTmp;
}
delete pHead;
pHead = pNewHead;
}
bool LinkList::IsEmpty()
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return false;
}

return pHead->pNext == NULL;
}

int LinkList::Length()
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return 0;
}
int nLength = 0;
LinkNode *pCur = pHead->pNext;
while (pCur)
{
nLength++;
pCur = pCur->pNext;
}

return nLength;
}
void LinkList::Reverse()
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
}
if (pHead->pNext == NULL)
{
cout << "链表为空!" << endl;
}
else
{
LinkNode *pCur = pHead->pNext->pNext;
LinkNode *pPre = pHead->pNext;
LinkNode *pnext = NULL;

while (pCur)
{
pnext = pCur->pNext;
pCur->pNext = pPre;

pPre = pCur;
pCur = pnext;
}

pHead->pNext->pNext = NULL;
pHead->pNext = pPre;
}
}
void LinkList::Destroy()
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return;
}

while (pHead->pNext)
{
LinkNode *pDel = pHead->pNext;
pHead->pNext = pDel->pNext;
delete pDel;
}
delete pHead;
pHead = NULL;
}
LinkNode* LinkList::Find(int nData)
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return NULL;
}
if (pHead->pNext == NULL)
{
cout << "链表为空!" << endl;
return NULL;
}

LinkNode *pCur = pHead->pNext;
while (pCur != NULL)
{
if (pCur->nValue == nData)
{
return pCur;
}
}

return NULL;
}
void LinkList::Print()
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return;
}
if (pHead->pNext == NULL)
{
cout << "链表为空!" << endl;
return;
}
else
{
LinkNode *pCur = pHead->pNext;
while (pCur)
{
cout << pCur->nValue << " ";
pCur = pCur->pNext;
}
cout << endl;
}
}
bool LinkList::IsLoop()
{
if (pHead == NULL)
{
cout << "链表未创建!" << endl;
return false;
}
if (pHead->pNext == NULL)
{
cout << "链表为空!" << endl;
return false;
}

LinkNode *pFast = pHead->pNext;
LinkNode *pLow = pHead->pNext;

while (pFast != NULL && pLow != NULL  && pFast->pNext != NULL)
{
pFast = pFast->pNext->pNext;
pLow = pLow->pNext;
if (pFast == pLow)
{
return true;
}
}
return false;
}

LinkNode* LinkList::FindByIndex(int n)
{
if (n <= 0 || n > Length())
{
return NULL;
}
LinkNode *pCur = pHead->pNext;
for (int i = 1; i < n; i++)
{
pCur = pCur->pNext;
}

return pCur;
}

LinkNode* LinkList::FindLastNum(int n)  //使用两个指针 高效操作
{
if (pHead == NULL || pHead->pNext == NULL || n < 0)
{
return NULL;
}
LinkNode *pCur = pHead->pNext;
LinkNode *pFirst = pHead->pNext;

while (n > 0)
{
pFirst = pFirst->pNext;
if (pFirst == NULL)
{
return NULL;
}
n--;
}
while (pFirst->pNext)
{
pFirst = pFirst->pNext;
pCur = pCur->pNext;
}

return pCur;
}
int main()
{
LinkList list;
list.Insert(12);
list.Insert(14);
list.Insert(2);
list.Insert(122);
list.Insert(5);
list.Insert(4);
list.Insert(7);
list.Print();
int k;
cout << "请输入要查询倒数第几个结点(计数从0开始):";
cin >> k;
LinkNode *p = list.FindByIndex(list.Length() - k);
if (p)
{
cout << p->nValue << endl;
}
LinkNode *p2 = list.FindLastNum(k);
if (p2)
{
cout << p2->nValue << endl;
}
system("pause");
return 0;
}