链表倒数第k个节点

题目：输入一个链表，输出该链表中倒数第k个结点

思路：1.遍历两遍链表，第一遍找到链表长度n，第二遍从0到n-k+1找到倒数第k个节点，不是最优解。

　　　2.使用两个指针，只需遍历一遍链表。第一个指针先走k-1步，然后两个指针同时向后遍历，当第一个指针走到尾节点时，第二个指针刚好在倒数第k个节点上。

注意：代码的鲁棒性！！

　　　链表为空，k为0，k大于链表长度的情况下应该返回NULL

思路1

#include <iostream>
#include <malloc.h>
using namespace std;

typedef struct Node
{
    int val;
    struct Node* next;
}ListNode;

ListNode* FindKthToTail(ListNode* pListHead, unsigned int k) {
    if(pListHead == NULL || k == 0)return NULL;
    ListNode* p = pListHead;
    ListNode* q = pListHead;
    int count = 0;
    while(p->next != NULL)
    {
        p = p->next;
        count++;
    }
    int n = count-k+1;
    if(n <= 0)return NULL;
    for(int i = 0;i < n;i++)
    {
        q = q->next;
    }
    return q;
}

int main()
{
    int n,num;
    ListNode* pHead = (ListNode*)malloc(sizeof(ListNode));
    pHead->next = NULL;
    cin>>n;
    ListNode* pTail = pHead;
    while(n--)
    {
        ListNode* p = (ListNode*)malloc(sizeof(ListNode));
        cin>>num;
        p->val = num;
        p->next = NULL;
        pTail->next = p;
        pTail = p;
    }
    ListNode* node = FindKthToTail2(pHead,3);
    cout<<node->val<<endl;
}

思路2

#include <iostream>
#include <malloc.h>
using namespace std;

typedef struct Node
{
    int val;
    struct Node* next;
}ListNode;

ListNode* FindKthToTail2(ListNode* pListHead, unsigned int k) {
    if(pListHead == NULL || k == 0)return NULL;
    ListNode* pAhead = pListHead;
    ListNode* pBehind = pListHead;
    for(unsigned int i = 0;i < k-1;i++)
    {
        if(pAhead->next != NULL)
            pAhead = pAhead->next;
        else{
            return NULL;
        }
    }
    while(pAhead->next != NULL)
    {

        pAhead = pAhead->next;
        pBehind = pBehind->next;
    }
    return pBehind;
}

int main()
{
    int n,num;
    ListNode* pHead = (ListNode*)malloc(sizeof(ListNode));
    pHead->next = NULL;
    cin>>n;
    ListNode* pTail = pHead;
    while(n--)
    {
        ListNode* p = (ListNode*)malloc(sizeof(ListNode));
        cin>>num;
        p->val = num;
        p->next = NULL;
        pTail->next = p;
        pTail = p;
    }
    ListNode* node = FindKthToTail2(pHead,3);
    cout<<node->val<<endl;
}