Single linked list to determine whether a single linked list has a ring

typedef struct LINKEDLIST
{
int data;
linkedlist* Pnext;
}linkedlist;

/**
* Test 6, write a function to check whether there are loop in a linked list
* @param plist [input] linked list
* @return BOOL, if there is loop inside, return true, otherwise false
*/
BOOL Checkloop (linkedlist* plist)
{
linkedlist* P1 = plist;
linkedlist* P2 = plist;

if (plist = null | | plist->pnext = NULL)
return false;

while (P2!= null && p2->pnext!= null)
{
P1 = p1->pnext;
P2 = p2->pnext->pnext;

if (P1==P2)
return true;
}
return false;
}