_142.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.ListNode;

/**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?*/
public class _142 {

    public ListNode detectCycle(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if ( slow == fast) {
                ListNode slow2 = head;
                while (slow2 != slow) {
                    slow = slow.next;
                    slow2 = slow2.next;
                }
                return slow;
            }
        }
        return null;
    }
}