题解 | 小红的二叉树

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    static int res = 0;
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        int n = in.nextInt();
        for (int i = n; i > 0; i--) {
            if (i != 1) {
                int temp = 1;
                if (i != 2) {
                    temp = 5;
                }
                for (int j = 0; j < n-i; j++) {
                    temp = (temp << 1) % 1000000007;
                }
                res = (res + temp) % 1000000007;
            }
        }
        System.out.println(res);

    }
}