19. Remove Nth Node From End of List

这个主要的知识点主要是1.双指针，2.在head前面用dummy的指针。

public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* slow = dummy;
        ListNode* fast = dummy;
        while(n > 0) {fast = fast->next; n--;}
        while(fast->next != NULL) {slow = slow->next; fast = fast->next;}
        slow->next = slow->next->next;
        return dummy->next;
    }
};