LeetCode/cpp/count-ways-to-build-good-strings.cpp

#include <cassert>
#include <vector>

class Solution {
  constexpr static int MOD = 1000000007;

  static void add_with_mod(std::vector<int>& good, std::size_t i, int value) {
    good[i] = (good[i] + value) % MOD;
  }

 public:
  int countGoodStrings(int low, int high, int zero, int one) {
    assert(low <= high);

    std::vector<int> good(high + 1, 0);
    good[0] = 1;

    for (int length = 1; length <= high; ++length) {
      if (length >= zero) {
        add_with_mod(good, length, good[length - zero]);
      }

      if (length >= one) {
        add_with_mod(good, length, good[length - one]);
      }
    }

    int total = 0;
    for (int i = low; i <= high; ++i) {
      total = (total + good[i]) % MOD;
    }

    return total;
  }
};

int main() {
  Solution s;

  assert(s.countGoodStrings(3, 3, 1, 1) == 8);
  assert(s.countGoodStrings(2, 3, 1, 2) == 5);

  return 0;
}