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2320. Count Number of Ways to Place Houses

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  int countHousePlacements(int n) {
    constexpr int kMod = 1'000'000'007;
    int house = 1;  // the number of ways ending in a house
    int space = 1;  // the number of ways ending in a space
    int total = house + space;

    for (int i = 2; i <= n; ++i) {
      house = space;
      space = total;
      total = (house + space) % kMod;
    }

    return static_cast<long>(total) * total % kMod;
  }
};

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class Solution {
  public int countHousePlacements(int n) {
    final int kMod = 1_000_000_007;
    int house = 1; // the number of ways ending in a house
    int space = 1; // the number of ways ending in a space
    int total = house + space;

    for (int i = 2; i <= n; ++i) {
      house = space;
      space = total;
      total = (house + space) % kMod;
    }

    return (int) ((long) total * total % kMod);
  }
}

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class Solution:
  def countHousePlacements(self, n: int) -> int:
    kMod = 1_000_000_007
    house = 1  # the number of ways ending in a house
    space = 1  # the number of ways ending in a space
    total = house + space

    for _ in range(2, n + 1):
      house = space
      space = total
      total = (house + space) % kMod

    return total**2 % kMod