STL版

#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#define int long long
using namespace std;
const int N = 40;
set<int> sf, sb;
vector<int> v;
int f[N];
int n, m;
int ans;

void DFS_Front(int u, int w) {
    if (u == n >> 1) {
        sf.insert(w);
        return;
    }
    DFS_Front(u + 1, w);
    DFS_Front(u + 1, (w + f[u + 1]) % m);
}

int bs(int t) {
    int l = 0, r = v.size() - 1;
    if (r < 0) return 0;
    while (l < r) {
        int mid = (l + r + 1) >> 1;
        if (v[mid] < t) l = mid;
        else r = mid - 1;
    }
    return v[l] < t ? v[l] : 0;
}

void DFS_Back(int u, int w) {
    if (u == n) {
        sb.insert(w);
        ans = max(ans, w + bs(m - w));
        return;
    }
    DFS_Back(u + 1, w);
    DFS_Back(u + 1, (w + f[u + 1]) % m);
}

signed main() {
    cin >> n >> m;
    for (int i = 1; i <= n; i++) cin >> f[i];

    DFS_Front(0, 0);
    v.assign(sf.begin(), sf.end());
    DFS_Back(n >> 1, 0);

    ans = max(ans, (*sf.rbegin() + *sb.rbegin()) % m);
    cout << ans << endl;
    return 0;
}

纯数组版

#include <iostream>
#include <algorithm>
#define int long long
using namespace std;
const int N = 40;
const int M = 1 << 18;
int sf[M], sfc;
int sb[M], sbc;
int f[N];
int n, m;
int ans;

void DFS_Front(int u, int w) {
    if(u == n >> 1) {
        sf[sfc++] = w;
        return;
    }
    DFS_Front(u + 1, w);
    DFS_Front(u + 1, (w + f[u + 1]) % m);
}

int bs(int t) {
    int l = 0, r = sfc - 1;
    while (l < r) {
        int mid = l + r + 1 >> 1;
        if (sf[mid] < t) l = mid;
        else r = mid - 1;
    }
    return sf[l] < t ? sf[l] : 0;
}

void DFS_Back(int u, int w) {
    if(u == n) {
        sb[sbc++] = w;
        ans = max(ans, w + bs(m - w));
        return;
    }
    DFS_Back(u + 1, w);
    DFS_Back(u + 1, (w + f[u + 1]) % m);
}

signed main () {
    cin >> n >> m;
    for (int i = 1; i <= n; i++) cin >> f[i];

    DFS_Front(0, 0);
    sort(sf, sf + sfc);
    sfc = unique(sf, sf + sfc) - sf;

    DFS_Back(n >> 1, 0);
    sort(sb, sb + sbc);

    ans = max(ans, (sf[sfc - 1] + sb[sbc - 1]) % m);
    cout << ans;
    return 0;
}