D - Joaozao, The Digit Maker

#include <bits/stdc++.h>
#define pb push_back
#define mk make_pair
#define fi first
#define se second
#define For(i,a,b) for(int (i)=(a);(i) < (b); ++(i))
using namespace std;
typedef vector<int> vi;
typedef pair<int,int> ii;
typedef long long ll;
typedef vector<bool> vb;
const int N=200010;

int n, v[N], b;

int main(void) {
    ios::sync_with_stdio(false);
    cin >> n >> b;
    multiset<int> q;

    for (int i = 0; i<2*n; i++) {
        int x;cin>>x;
        q.insert(x);
    }

    vi ans;

    multiset<int>::reverse_iterator it = q.rbegin();

    vector<int> qq;

    while (!q.empty()) {
        int x = *q.rbegin();

        q.erase(q.find(x));

        auto itx = q.upper_bound(b-x-1);

        if (itx == q.begin()) {
            qq.pb(x);
        }
        else {
            itx--;
            ans.pb(x + *itx);
            q.erase(itx);
        }
    }

    sort(qq.begin(), qq.end());

    for (int i = qq.size()-1; i>=0; i-=2) {
        ans.pb(qq[i]+qq[i-1]-b);
    }
    sort(ans.rbegin(), ans.rend());

    for (int i = 0; i<ans.size(); i++)
        cout << ans[i] << " ";
    cout << '\n';

    return 0;
}