/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val) { $this->val = $val; }
 * }
 */

class Solution {
    /**
     * @param ListNode $head
     * @return ListNode
     */
    function detectCycle($head) {
        if($head == null || $head->next == null) return null;
        $fast = $head;
        $slow = $head;
        
        while ($fast != null && $fast->next != null) {
            $fast = $fast->next->next;
            $slow = $slow->next;
            if ($fast === $slow) {
                break;
            }
        }

        $ptr = $head;
        while ($ptr !== $slow) {
            $ptr = $ptr->next;
            $slow = $slow->next;
        }

        return $ptr;
    }
}