第四题： import java.util.Scanner; public class Problem4 { private static void solution(int k) { long[][] recorder = new long[4][k + 1]; recorder[0][0] = 1; long mod = 1000000007; for (int i = 1; i < k + 1; ++ i) { for (int j = 0; j < 4; ++ j) { for (int m = 0; m < 4; ++ m) { if (m == j) { continue; } recorder[j][i] += recorder[m][i - 1] % mod; recorder[j][i] %= mod; } recorder[j][i] %= mod; } } System.out.println(recorder[0][k] % mod); } public static void main(String[] args) { Scanner scanner = new Scanner(System.in); int k = scanner.nextInt(); solution(k); } }