CareerCup

add addresses of the node in hast table and every time check the table when new node is accessed if address value all ready exist then there is loop in the list
/////============code
ptr=strat;
while(ptr!=NULL)
{
if(hash_has(ptr))
return "LOOP"
else
hash_insert(ptr);///insert into hash
ptr=ptr->nxt;
}
return "NO_LOOP"
=========================================

make to pointers..
increase the first one by 1 and the second one by 2 nodes..
where ever they are equal or fast pointer overtakes second one => loop detected.

while(fast->next==null)
{
 if(fast == slow || fast->next == slow)
  {//loop detected}
}

This is incorrect and incomplete

walk through the linked list {

if current node exists in idSet{ return true;}
else idSet.add(current node's id)
}

should be o(n).

Use Floyd cyclic detection algorithm, Have two pointers one is slow and other is fast.
Here slow iterates with one step and fast iterates with two steps.
At any time, when slow is equal to fast, terminate the algorithm.

Worse case is - O(n)