loj上AC，洛谷31分求助

rt

代码如下

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
const int N = 22;

ll a[N], s[1 << N];
int n, len = 0; ll m, ans = 0;

void dfs1(int u, ll now) {
    if(now > m) return;
    if(u > (n - (n >> 1))) {
        s[++len] = now;
        return;
    }
    dfs1(u + 1, now);
    dfs1(u + 1, now + a[u]);
}

void dfs2(int u, ll now) {
    if(now > m) return;
    if(u > n) {
        int l = 1, r = len;
        while(l < r) {
            int mid = (l + r + 1) >> 1;
            if(s[mid] + now <= m) l = mid;
            else r = mid - 1;
        }
        ans += (ll)(l);
        return;
    }
    dfs2(u + 1, now);
    dfs2(u + 1, now + a[u]);
}

int main() {
    cin >> n >> m;
    for(int i = 1; i <= n; i++) cin >> a[i];
    sort(a + 1, a + n + 1);
    if(n == 1) {
        if(a[1] <= m) puts("2");
        else puts("1");
        return 0;
    }
    dfs1(1, 0ll);
    sort(s + 1, s + len + 1);
    dfs2(n - n / 2 + 1, 0ll);
    cout << ans << endl; 
    return 0;
} 

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