19. 删除链表的倒数第 N 个结点

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* x=new ListNode(0);
        x->next=head;
        ListNode *f=x;
        ListNode *s=x;
        n=n+1;
        while(n--){
            f=f->next;
        }
        while(f){
            s=s->next;
            f=f->next;
        }
        s->next=s->next->next;
        return x->next;
    }
};