解析

class Solution {
public:
  ListNode *removeNthFromEnd(ListNode *head, int n) {
    if (head->next == nullptr) {
      return nullptr;
    }
    ListNode *dummy = new ListNode();
    dummy->next = head;
    ListNode *slow = dummy;
    ListNode *fast = dummy;
    for (int i = 0; i <= n; ++i) {
      fast = fast->next;
    }
    while (fast != nullptr) {
      fast = fast->next;
      slow = slow->next;
    }
    slow->next = slow->next->next;
    return dummy->next;
  }
};