Detect Loop in linked list

problem id:--http://practice.geeksforgeeks.org/problems/detect-loop-in-linked-list/1

code:--

int detectloop(struct node *list){
struct node *p,*q;
p=list,q=list;
int flag=1;
while(flag==1)
    {
    if(p==NULL||p->next==NULL ||q==NULL||(q->next)==NULL||(q->next)==NULL)  
        {
        flag=0;
        break;
        }
   
   
    p=p->next;
    q=(q->next)->next;
    //printf("%d   %d \n",p->data,q->data);
    if(p==q)
        break;
    }
if(flag==1)
    return 1;
else
    return 0;
}