题解 | #神奇的口袋#

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <stack>
#include <map>
#include <queue>
using namespace std;

int arr[30];
int _count = 0;
int n;

void select(int i, int weight) { //选到了第i个位置，此时的容量为weight
    if (weight == 40) {
        _count++;
        return;
    }
    if (weight > 40 || i >= n) { //重量超过了，或者越界
        return;
    }
    //可以选择拿第i个物品，或者不拿
    select(i + 1, weight + arr[i]);
    select(i + 1, weight);
    return;
}

int main() {
    while (scanf("%d", &n) != EOF) {
        for (int i = 0; i < 30; i++) {
            arr[i] = 0;
        }
        for (int i = 0; i < n; i++) {
            scanf("%d", &arr[i]);
        }
        select(0, 0);
        printf("%d\n", _count);
    }

}