<?php function hasLoop($head) { $slow = $head; $fast = $head; while ($fast !== null && $fast->next !== null) { $slow = $slow->next; $fast = $fast->next->next; if ($slow === $fast) { return true; } } return false; } // example usage class Node { public $data; public $next; public function __construct($data) { $this->data = $data; $this->next = null; } } $a = new Node(1); $b = new Node(2); $c = new Node(3); $d = new Node(4); $a->next = $b; $b->next = $c; $c->next = $d; var_dump(hasLoop($a)); // false $d->next = $b; var_dump(hasLoop($a)); // true
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