写一个删除链表的倒数第n个节点的问题能通过，但是提交答案显示出错

Question

编程小白，写一个删除链表的倒数第n个节点的问题能通过，但是提交答案显示出错，求大神解答一下，谢谢

/**

• Definition for singly-linked list.
• struct ListNode {
• int val;
• ListNode *next;
• ListNode(int x) : val(x), next(NULL) {}

• };
  /
  class Solution {
  public:
  ListNode removeNthFromEnd(ListNode* head, int n){
  int i;
  ListNode *p1=head->next;
  ListNode *p=head;;
  while(p1 !=NULL && n>0){
  p1=p1->next;
  --n;
  }
  while(p1 !=NULL) {
  p1=p1->next;
  p=p->next;
  };

  p->next=p->next->next;
  return head;
  }
  

  };
  错误；Line 24: member access within null pointer of type 'struct ListNode'

Answer 1

你需要判断先走的指针是不是空指针
class Solution {
public:
ListNode* removeNthFromEnd(ListNode* head, int n) {
ListNode* fast = head;
ListNode* slow = head;
while(n>0) {
fast = fast -> next;
n--;
}
if(fast == NULL) {
if(head == NULL) {
return head;
} else {
return head -> next;
}
}
while(fast -> next) {
fast = fast -> next;
slow = slow -> next;
}
slow -> next = slow -> next -> next;
return head;
}
};