链表中倒数第k个结点

题目
　输入一个链表，输出该链表中倒数第k个结点．为了符合大多数人的习惯，本题从１开始计数，即链表的尾节点是倒数第一个结点．

双指针实现一次遍历就能找到，要注意代码的鲁棒性．

code:

ListNode* FindKthToTail(ListNode* pListHead, unsigned int k)
{
    if (pListHead == NULL || k == 0)
    {
        return NULL;
    }
    ListNode* pFirst = pListHead;
    ListNode* pSecond = NULL;
    for (unsigned int i = 0; i < k - 1; ++i)
    {
        if (pFirst->m_pNext != NULL)
        {
            pFirst = pFirst->m_pNext;
        }
        else
        {
            return NULL;
        }
    }

    pSecond = pListHead;

    while (pFirst->m_pNext != NULL) 
    {
        pFirst = pFirst->m_pNext;
        pSecond = pSecond->m_pNext;
    }

    return pSecond;
}

测试代码：

#include <iostream>
using namespace std;
struct ListNode
{
    int       m_nValue;
    ListNode* m_pNext;
    ListNode(int n) : m_nValue(n), m_pNext(NULL) {}
};

ListNode* FindKthToTail(ListNode* pListHead, unsigned int k)
{
    if (pListHead == NULL || k == 0)
    {
        return NULL;
    }
    ListNode* pFirst = pListHead;
    ListNode* pSecond = NULL;
    for (unsigned int i = 0; i < k - 1; ++i)
    {
        if (pFirst->m_pNext != NULL)
        {
            pFirst = pFirst->m_pNext;
        }
        else
        {
            return NULL;
        }
    }

    pSecond = pListHead;

    while (pFirst->m_pNext != NULL) 
    {
        pFirst = pFirst->m_pNext;
        pSecond = pSecond->m_pNext;
    }

    return pSecond;
}

ListNode* CreateList()
{
    int length;
    cout << "Enter the length of linklist:";
    cin >> length;
    int number;
    cout << "Enter the value of each number:";
    ListNode* pHead = NULL;
    ListNode* pTmp = NULL;
    for (int i = 0; i < length; i++)
    {
        cin >> number;
        ListNode* s = new ListNode(number);

        if (!pHead)
        {   
            pHead = s;
            pTmp = s;
        }

        else
        {
            pTmp->m_pNext = s;
            pTmp = s;
        }
    }

    return pHead;
}

void PrintLinkList(ListNode* pHead)
{
    if (pHead == NULL)
        return;
    while (pHead)
    {
        cout << pHead->m_nValue <<"-->";
        pHead = pHead->m_pNext;
        if (pHead->m_pNext == NULL)
        {
            cout << pHead->m_nValue <<"^";
            break;
        }
    }
    cout << endl;
}
void Test()
{
    ListNode* p = CreateList();
    PrintLinkList(p);
    int k;
    cout << "Enter k: ";
    cin >> k;
    ListNode* pTest = FindKthToTail(p,k);
    if (pTest)
    {
        cout << "the kth value to tail is: "<<pTest->m_nValue<<endl;
    }
    else
    {
        cout << "Error!" <<endl;
    }
}

int main()
{
    Test();
    return 0;
}