单链表有环，是指单链表的最后一个节点的指针指向了链表中的某个结点（通常单链表的最后一个节点的指针域是空的）。试编写算法判断单链表是否存在环。

#include <iostream>typedef  struct node{int data;node* next;
}node,*list;list Init()
{list head=(list) malloc(sizeof (node));head->next= nullptr;head->data=-1;return head;
}list Buynewnode(int x)
{list tmp=new node;tmp->next= nullptr;tmp->data=x;return tmp;
}void print(list head)
{for(list pointer=head->next;pointer!= nullptr;pointer=pointer->next) {printf("%3d",pointer->data);}puts("");
}void is_cir(list head)
{list slow=head,fast=head;while(fast){slow=slow->next;fast=fast->next;if(fast) {fast=fast->next;}else break;if(fast==slow) {printf("there is a circle\n");return ;}}printf("there is no circle\n");return;
}
int main() {list head=Init();list pointer=head;list record= nullptr;//制造环for(int i=0;i<10;i++){if(i==6) record=pointer;pointer=pointer->next= Buynewnode(i+1);}pointer->next=record;is_cir(head);list head2=Init();pointer=head2;record= nullptr;for(int i=0;i<10;i++){pointer=pointer->next= Buynewnode(i+1);}is_cir(head2);return 0;
}