*[hackerrank]Girlfriend Necklace

https://www.hackerrank.com/contests/w8/challenges/gneck

有点意思。是DP，最优解包含最优子问题。F(X)=F(X-1)+F(X-3)。因为F(X-1)右边放个“R”和F(X-3)右边放"RRB"是左右可以互推的，包含全了最右是R和B的两种情况。

#include <iostream>
using namespace std;const uint64_t MOD = 1000000007;uint64_t resolve(uint64_t x) {uint64_t a = 3; //2uint64_t b = 4; //3uint64_t c = 6; //4if (x == 2)return a;if (x == 3)return b;if (x == 4)return c;for (int64_t i = 5; i <= x; i++) {int64_t r = a + c;a = b % MOD;b = c % MOD;c = r % MOD;}return c;
}int main() {int T;cin >> T;while (T--) {uint64_t N;cin >> N;cout << resolve(N) << endl;}return 0;
}