#142 Linked List Cycle II.java

//https://leetcode.com/problems/linked-list-cycle-ii/
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
//O(N) Space used.
// public class Solution {
//     public ListNode detectCycle(ListNode head) {
//         HashSet<ListNode> set = new HashSet<>();
//         while(head!=null) {
//             if(set.contains(head)) return head;
//             set.add(head);
//             head = head.next;
//         }
//         return null;
//     }
// }

// O(1) Space
/**
 * 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 slow = head;
        ListNode fast = head;
        while(fast!=null && fast.next!=null) {
            slow = slow.next;
            fast=fast.next.next;
            if(slow == fast) break;
        }
        if(fast == null || fast.next == null) return null;
        slow = head;
        while(slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }
}