力扣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* dummyhead = new ListNode(0, head);
        ListNode* slow = dummyhead;
        ListNode* fast = dummyhead;
        while(n-- && fast->next) {
            fast = fast->next;
        }
        while(fast->next) {
            fast = fast->next;
            slow = slow->next;
        }
        ListNode* tmp = slow->next;
        slow->next = tmp->next;
        delete tmp;
        tmp = dummyhead->next;
        delete dummyhead;
        return tmp;
    }
};