linked-list-cycles.java

public class Node {
	protected String data;
	protected Node next;
    
    public Node(final String data, final Node next) {
        this.data = data;
        this.next = next;
    }
    
    public Node getNext() {
    	return next;   
    }
    
    public void setNext(final Node next) {
    	this.next = next;   
    }
    
    public String getData() {
    	return this.data;   
    }
    
    public static boolean isThereLoop(final Node head) throws Exception {
    	boolean stop = false;
        boolean tick = false;
        Node fast_current = head;
        Node slow_current = head;
        int count = 0;
        while (!stop || count > 10000) {
        	fast_current = fast_current.getNext();
            
            if (tick) {
                slow_current = slow_current.getNext();
            }
            
            if (fast_current == null) {
            	return false;
            } else if (fast_current.equals(slow_current)) {
                return true;
            }
            
            tick = !tick;
            count++;
        }
                       
        throw new Exception("failure!!!");
    }
    
    public static void main(String[] args) {
        //setup linked list
    	final Node head = new Node("head", null);
        final Node next1 = new Node("1", null);
        head.setNext(next1);
        final Node next2 = new Node("2", null);
        next1.setNext(next2);
        final Node next3 = new Node("3", null);
        next2.setNext(next3);
        final Node next4 = new Node("4", null);
        next3.setNext(next4);
        next4.setNext(next2); // the loop!
        
        try {
        	System.out.println(isThereLoop(head));   
        } catch (Exception e) {
            System.out.println("There was a problem");
        }
    }
}