2.6

detectcircular.java

/**
 * Created by Readman on 6/23/14.
 */
public class detectcircular {
    // time, if there is no circular, it will never stop. else n
    // space, 1
    public static ListNode detectcircular(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;

        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast)
                break;
        }
        fast = slow;
        slow = head;
        while (fast != slow) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }

    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next= new ListNode(5);
        head.next.next.next.next.next = head.next.next;
        System.out.println(detectcircular(head).val);
    }

}