Java题解 | 小红的二叉树

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int p = 1000000007;
        if (n == 1) System.out.println(0);
        else if (n == 2) System.out.println(1);
        else {
            long ans = 1, x = 1;
            n -= 2;
            while (n-- > 0) {
                x = 2 * x % p;
                ans = (ans + 3 * x) % p;
            }
            System.out.println(ans);
        }
        in.close();
    }
}