141. Linked List Cycle

Return true if the linked list has a cycle. In the input, pos is the index the tail links back to (-1 means no cycle).

Move slow one step and fast two steps at a time. In a cycle, fast gains one step per move and must land on slow; otherwise fast falls off the end.

1function hasCycle(head: ListNode | null): boolean {
2  let slow = head, fast = head;
3  while (fast && fast.next) {
4    slow = slow!.next;
5    fast = fast.next.next;
6    if (slow === fast) return true;
7  }
8  return false;
9}

moves = 0

list

slow,fast

→

-4

↪ 2

Both start at head.

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Space: play/pause · ←/→: step