FindLoopHead.java

import java.util.*;

public class FindLoopHead {

	public static void main(String[] args) {
		MySingleList<Integer> l1 = new MySingleList<Integer>(new Integer[] {1,2,3,4,5,3});
        l1.print();
        System.out.println("The loop head is: " + findHead(l1));
	}
	
	/**
	 * find the loop head by using hashset
	 * @param l
	 * @return
	 */
	public static int findHead(MySingleList<Integer> l){
		Set<Integer> s = new HashSet<Integer>();
		Node<Integer> curr = l.head;
		while(true){
			if(!s.contains(curr.val)){
				s.add(curr.val);
				curr = curr.next;
			}else{
				return curr.val;
			}
		}
	}
}

you cannot store the value of the node, because different node may have the same value

for example:
1 -> 2-> 4 -> 3 -> 2
is not a circle LinkedList even there exists two node have the same value 2