1015. Smallest Integer Divisible by K.py

class Solution:
  def smallestRepunitDivByK(self, k: int) -> int:
    if k % 10 not in {1, 3, 7, 9}:
      return -1

    seen = set()
    n = 0

    for length in range(1, k + 1):
      n = (n * 10 + 1) % k
      if n == 0:
        return length
      if n in seen:
        return -1
      seen.add(n)

    return -1