0142. Linked List Cycle II.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 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;

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

        return head;
    }
}