/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */
/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 * @author Senky
 * @date 2023.04.18
 * @par url https://www.nowcoder.com/creation/manager/content/584337070?type=column&status=-1
 * @brief 
     (1)k为0返回NULL
     (2)k大于链表长度返回NULL
     (3)k等于链表长度返回头结点
 * @param pHead ListNode类 
 * @param k int整型 
 * @return ListNode类
 */
struct ListNode* FindKthToTail(struct ListNode* pHead, int k ) {
    // write code here
    struct ListNode* first =pHead;
    struct ListNode* second =pHead;
    struct ListNode* result = NULL;
    int count = 0;

    /*Count the length of the chain table by variable count*/
    while(first)
    {
        count++;
        first = first->next;
    }

    if(0 == k)          result = NULL;
    else if(k > count)  result = NULL;
    else if(k == count) result = pHead;
    
    for(int i = 0; i < count -k; i++)
    {
        second = second->next;
        result = second;
    }

    return result;
}

Algorithm:

倒数第k个,就是正数第n-k+1个,从0开始就是输出第n-k个