/**
 * 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
 * @param head ListNode类 
 * @param n int整型 
 * @return ListNode类
 */
struct ListNode* removeNthFromEnd(struct ListNode* head, int n ) {
    // write code here
    struct ListNode* first =head;
    struct ListNode* second =head;
    struct ListNode* result =head;
    struct ListNode* pre = NULL;

    int count = 0;

    while(first)
    {
        count++;
        first = first->next;
    }

    if(1 == count) result = NULL;
    if(count < n)  result = NULL;
    if(count == n) result = head->next;

    for(int i = 0; i < count - n; i++)
    {
        pre = second;
        second = second->next;
    }
    if(pre) pre->next = second->next;

    return result;
}

Algorithm:

删除倒数第k个结点,首先找到正数第n-k+1结点,在循环的时候用一个指针记录其前驱，找到第n-k+1个结点时也找到了第n-k个结点，修改指针域即可删除第n-k+1个结点;