LinkedListCycle.java

package LinkedList;

//  Given head, the head of a linked list, determine if the linked list has a cycle in it.
//  There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer.
//  Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.
//  Return true if there is a cycle in the linked list. Otherwise, return false.
public class LinkedListCycle {

    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;

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

            if (fast == slow) {
                return true;
            }
        }

        return false;
    }

    public int lengthOfCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;

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

            if (fast == slow) {

                ListNode temp = slow;
                int length = 0;

                do {
                    temp = temp.next;
                    length++;
                } while (temp != slow);

                return length;
            }
        }

        return 0;
    }

}


class ListNode {

    int val;
    ListNode next;

    ListNode() {
    }

    ListNode(int x) {
        val = x;
        next = null;
    }

}