找出链表中倒数第 k 个结点

/*
题目:输入一个单向链表,输出该链表中倒数第 k 个结点。链表的倒数第 0 个结点为链表
的尾指针。
链表结点定义如下:
struct node
{
    int data;
    struct node *next;
};
typedef struct node* PtrToNode; 
typedef PtrToNode Position;
typedef Position List;
*/
#include <iostream>
#include <assert.h>
#include "./List.h"
using namespace std;
void print_list(List list)
{
    assert(list != NULL);
    list = First(list);
    while(list != NULL)
    {
        cout<<list->data<<" ";
        list = list->next;
    }
    cout<<endl;
}
Position get_last_kth(List list, int k)
{
    assert(list != NULL);
    Position tmp = First(list);
    while(tmp != NULL && --k >= 0)
        tmp = tmp->next;
    if(NULL == tmp)//the length of list <= k
        return NULL;
    Position ret = First(list);
    while(tmp != NULL)
    {
        ret = ret->next;
        tmp = tmp->next;
    }
    return ret;
}
int main(int argc, char const *argv[])
{
    List list = CreateEmptyList();
    int k;
    for(int i = 1; i != 20; ++i)
        PushBack(i, list);
    print_list(list);
    while(cin>>k)
    {
        Position tmp = get_last_kth(list, k);
        cout<<"k:"<<k<<" = ";
        if(tmp == NULL)
            cout<<"NULL";
        else
            cout<<tmp->data;
        cout<<endl;
    }
    return 0;
}