迭代法

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNext()) {
            int month = scanner.nextInt();
            System.out.println(f(month));
        }
    }

    public static int f(int month) {
        if (month < 3) {
            return 1;
        }
        // 前两个月的情况已经写出来了，我们从第三个月开始算
        int m1 = 0;
        int m2 = 1;
        int m3 = 0;
        for (int i = 2; i < month; i++) {
            m3 = m1 + m2;
            m2 = m1;
            m1 = m3;
        }
        return m1 + m2 + m3;
    }
}

递归法

找规律，累加，斐波那契数列，青蛙跳台阶。递归代码很简单，交给计算机去递归就好了

    public static int f(int month) {
        if (month < 3) {
            return 1;
        }
        return (f(month-2)+f(month-1));
    }