141.linked-list-cycle.ts

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

function hasCycle(head: ListNode | null): boolean {
  const set = new Set<ListNode>();

  while (head) {
    if (set.has(head)) {
      return true;
    }
    set.add(head);
    head = head.next;
  }

  return false;
}

function hasCycle2(head: ListNode | null): boolean {
  let slow = head;
  let fast = head?.next;

  while (slow && fast) {
    if (slow === fast) {
      return true;
    }
    slow = slow.next;
    fast = fast.next?.next;
  }

  return false;
}