19.删除链表的倒数第 N 个结点 (Medium)*

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode *fast = head;
        while(n--) {
            fast = fast->next;
        }
        if(fast == nullptr) {
            return head->next;
        }
        ListNode *slow = head;
        while(fast->next != nullptr) {
            slow = slow->next;
            fast = fast->next;
        }
        slow->next = slow->next->next;
        return head;
    }
};