Solution.java

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode fast=head;
        ListNode slow=head;
        while (true) {
            if (fast==null||fast.next==null) {
                return null;
            }
            fast=fast.next.next;
            slow=slow.next;
            if (fast==slow) {
                ListNode again=head;
                while (true) {
                    if (slow==again) {
                        return again;
                    }else {
                        slow=slow.next;
                        again=again.next;
                    }
                }
            }
        }
    }
}