问备案授权码？

boolean hasLoop(Node first) {

    if(first == null) // list does not exist..so no loop eithe

        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;

    }

}

boolean hasLoop(Node first) {

    Node slow = first;

    Node fast = first;

    while(fast != null && fast.next != null) {

        slow = slow.next;          // 1 hop

        fast = fast.next.next;     // 2 hops 

        if(slow == fast)  // fast caught up to slow, so there is a loop

            return true;

    }

    return false;  // fast reached null, so the list terminates

}