判断一个单向链表中是否有环

struct listNode{
int  data;
struct listNode *next
}list;

int find_cicle(list * head){

listNode *pfast = head;
listNode *pslow = head;

if(pfast==null){
return -1
}

while(pfast&&pfast->next){

pfast=pfast->next->next;
pslow=pslow->next;
if(pfast==pslow){
return 1;
}
}
}