How to detect a loop in a linked list?

You can make use of Floyd’s cycle-finding algorithm, also known as tortoise and hare algorithm.

The idea is to have two references to the list and move them at different speeds. Move one forward by 1 node and the other by 2 nodes.

• If the linked list has a loop they
  will definitely meet.
• Else either of
  the two references(or their next)
  will become null.

Java function implementing the algorithm:

boolean hasLoop(Node first) {

    if(first == null) // list does not exist..so no loop either
        return false;

    Node slow, fast; // create two references.

    slow = fast = first; // make both refer to the start of the list

    while(true) {

        slow = slow.next;          // 1 hop

        if(fast.next != null)
            fast = fast.next.next; // 2 hops
        else
            return false;          // next node null => no loop

        if(slow == null || fast == null) // if either hits null..no loop
            return false;

        if(slow == fast) // if the two ever meet...we must have a loop
            return true;
    }
}