LeeCode-Linked List Cycle

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

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     struct ListNode *next;
  * };
  */
 bool hasCycle(struct ListNode *head)
 {
     struct ListNode *p1;
     struct ListNode *p2;

     p1=head;
     p2=head;

     if(p1==NULL||p1->next==NULL)
         return false;

     while(p2->next!=NULL&&p2->next->next!=NULL)
     {
         p1=p1->next;
         p2=p2->next->next;

         if(p1==p2)
             return true;
     }

     return false;
 }