题解（C++ PYTHON JAVA）

这道题是个基本题，十分的简单。
所以，我们要用3种语言去做，巩固基本知识。

先来了解基本语法：
传送门

C++实现

#include<bits/stdc++.h>
using namespace std;
long long s[105];
int main() {
	s[1]=1;
	s[2]=2;
	int n;
	cin>>n;
    s[3]=4;
	for(int i=4;i<=n;i++){
		s[i]=s[i-1]+s[i-2]+s[i-3];
	}
    cout<<s[n];
	return 0;
}

PY实现：

# Initialize the s array with initial values
s = [0] * 105
s[1] = 1
s[2] = 2
s[3] = 4

# Take input for n
n = int(input())

# Calculate the values of s from 4 to n
for i in range(4, n + 1):
    s[i] = s[i - 1] + s[i - 2] + s[i - 3]

# Print the value of s at index n
print(s[n])

java实现：

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        long[] s = new long[105];
        s[1] = 1;
        s[2] = 2;
        s[3] = 4;

        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();

        for (int i = 4; i <= n; i++) {
            s[i] = s[i - 1] + s[i - 2] + s[i - 3];
        }

        System.out.println(s[n]);
    }
}