Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

#include<stdio.h>
#include<stdlib.h>

typedef struct ListNode {
    int val;
    struct ListNode *next;
 }*ListNode;

bool hasCycle(ListNode *head) {
     ListNode *p1,*p2;
    if(head==NULL||head->next==NULL) return false;
    if(head->next==head) return true;
    for(p1=p2=head;p1!=NULL&&p1->next!=NULL;){
        p1=p1->next;
        p1=p1->next;
        p2=p2->next;
        if(p1==p2) return true;
    }
    return false;
}
 

Linked List Cycle