hasCycle.js

/**
 * @param {ListNode} head
 * @return {boolean}
 */
function hasCycle(head) {
	if (!head) return false;

	let [fast, slow] = [head, head];

	while (slow.next && fast.next && fast.next.next) {
		slow = slow.next;
		fast = fast.next.next;

		if (fast === slow) return true;
	}

	return false;
}

module.exports = hasCycle;