Detect Cycle in Linked List

Given the head of a linked list, determine whether the list contains a cycle.
Use the Floyd's Tortoise and Hare algorithm for optimal performance.

Input:  head = [3, 2, 0, -4], pos = 1  
Output: true  
Explanation: The tail connects to the node at position 1.

export class ListNode {
  val: number;
  next: ListNode | null;

  constructor(val?: number, next?: ListNode | null) {
    this.val = val ?? 0;
    this.next = next ?? null;
  }
}

export function hasCycle(head: ListNode | null): boolean {
  let slow = head;
  let fast = head;

  while (fast !== null && fast.next !== null) {
    slow = slow.next!;         // moves 1 step
    fast = fast.next.next;     // moves 2 steps

    if (slow === fast) return true; // cycle detected
  }

  return false;
}