How to check whether a linked list is circular ?

Question

Can someone help me with algorithm and code?

Answer 1

Algorithm
Step 1: Take 2 pointers travelling at different speeds start from the head of the linked list called faster pointer and slower pointer.
Step 2: Iterate through a loop.
Step 3: If the faster pointer reaches a NULL pointer then return saying that the list is acyclic and not circular.
Step 4: If the faster pointer is ever equal to the slower pointer or the faster pointer's next pointer is ever equal to the slower pointer then return that the list is circular.
Step 5: Advance the slower pointer one node and faster pointer by 2 nodes.

Code

bool is_circular(Node *head)
{
   Node *slower, * faster;
   slower = head;
   faster = head->next; //start faster one node ahead

   while(true) {
     // if the faster pointer encounters a NULL element
     if( !faster || !faster->next)
       return false;
    //if faster pointer ever equals slower or faster's next
    //pointer is ever equal to slow then it's a circular list
     else if (faster == slower || faster->next == slower)
        return true;
     else{
       // advance the pointers
        slower = slower->next;
        faster = faster->next->next;
      }
   }
}