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1015. Smallest Integer Divisible by K

  • Time: $O(k)$
  • Space: $O(k)$
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class Solution {
 public:
  int smallestRepunitDivByK(int k) {
    if (k % 10 != 1 && k % 10 != 3 && k % 10 != 7 && k % 10 != 9)
      return -1;

    unordered_set<int> seen;
    int n = 0;

    for (int length = 1; length <= k; ++length) {
      n = (n * 10 + 1) % k;
      if (n == 0)
        return length;
      if (seen.contains(n))
        return -1;
      seen.insert(n);
    }

    return -1;
  }
};

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class Solution {
  public int smallestRepunitDivByK(int k) {
    if (k % 10 != 1 && k % 10 != 3 && k % 10 != 7 && k % 10 != 9)
      return -1;

    Set<Integer> seen = new HashSet<>();
    int n = 0;

    for (int length = 1; length <= k; ++length) {
      n = (n * 10 + 1) % k;
      if (n == 0)
        return length;
      if (seen.contains(n))
        return -1;
      seen.add(n);
    }

    return -1;
  }
}

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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