上楼梯 牛客练习题

上楼梯（斐波那契数的高级用法） 牛客练习题

import java.util.Scanner;

public class 上楼梯 {
    public int  countWays(int n){
        int a = 1,b = 2,c = 4;
        if (n== 1){
            return a;
        }else if (n == 2){
            return b;
        }else if (n == 3){
            return c;
        }else {
            for (int i = 4 ;i <= n;i++){
                int temp = ((a+b)%1000000007+c)%1000000007;
                a = b;
                b = c;
                c = temp;
            }
            return c;
         }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        上楼梯 a = new 上楼梯();
        System.out.println("返回："+a.countWays(n));
    }
}