IsHaveCircle.java

package algorithm;

public class IsHaveCircle {

    static class Node {
        Node next;
        int value;

        public Node(int value) {
            this.value = value;
        }
    }

    public static int isHaveCircleAndMeetWhere(Node head) {
        if (head == null || head.next == null) {
            return -1;
        }
        Node a = head;
        Node b = head;
        while (b.next.next != null) {
            a = a.next;
            b = b.next.next;

            if (a == b) {
                Node c = head;
                while (c != a) {
                    a = a.next;
                    c = c.next;
                }
                return c.value;
            }
            a = a.next;
            b = b.next.next;
        }
        return -1;
    }

    public static void main(String[] args) {
        Node n1 = new Node(1);
        Node n2 = new Node(2);
        Node n3 = new Node(3);
        Node n4 = new Node(4);
        Node n5 = new Node(5);
        Node n6 = new Node(6);

        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n4;

        System.out.println(isHaveCircleAndMeetWhere(n1));
    }

}