LinkedListCycleII142.java

/**
 * Given a linked list, return the node where the cycle begins.
 * If there is no cycle, return null.
 *
 * Note: Do not modify the linked list.
 *
 * Follow up:
 * Can you solve it without using extra space?
 */

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */

public class LinkedListCycleII142 {
    public ListNode detectCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;

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

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

        return null;
    }
}