C :

#include <stdio.h>
#include <string.h>
int time;
int stuff[20];

void check(int sum, int i, int n)
{
    if(i == n+1) // fail
        return;
    else if(sum == 40){
        time++;
        return;
    }else if(sum < 40){
        check(sum, i+1, n);
        check(sum+stuff[i], i+1, n);
    }else if(sum > 40)
        return;

    return ;
}

int main()
{
    int n;
    while(scanf("%d", &n) != EOF){
        memset(stuff, 0, sizeof(stuff));
        time=0;
        for(int i = 0; i < n; i++)
            scanf("%d", stuff + i);
        check(0, 0, n);
        printf("%d\n", time);
    }
    return 0;
}

C++ :

#include<iostream>
#include<stdio.h>
#include<algorithm>

using namespace std;

int a[20];
int n;

void compute(int a[],int &sum,int k,int left){//递归函数
    //cout<<"left="<<left<<endl;
    if(left==0){
        sum++;
        return;
    }
    else{
        if(k>=n||a[k]>left)return;
        else{
            for(int i=k;i<n;i++)
                compute(a,sum,i+1,left-a[i]);
        }
    }
}
    

int main(void){
    int sum;
    while(scanf("%d",&n)!=EOF){
        sum=0;
        for(int i=0;i<n;i++)
                scanf("%d",&a[i]);
        sort(a,a+n);
        compute(a,sum,0,40);
        printf("%d\n",sum);
    }
    return 0;
}

Java :

import java.io.BufferedInputStream;
import java.util.Arrays;
import java.util.Scanner;
public class Main{
    static int n;
    static int arr[];
    public static void main(String[] args) {
        Scanner s = new Scanner(new BufferedInputStream(System.in));
        while(s.hasNextInt()){
            n = s.nextInt();
            arr = new int[n];
            for(int i=0;i<n; i++)
                arr[i] = s.nextInt();
            System.out.println(f(n-1,40));
        }
    }

    static int f(int n,int sum){
        if(sum < 0)
            return 0;
        if(sum == 0)
            return 1;
        if(n==0)
            return arr[0] == sum ? 1:0;
        else
            return f(n-1,sum)+f(n-1,sum-arr[n]);
    }
}