Submission #6363770

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
#define Int int64_t
#define dump(x) cout << (x) << endl
#define fi first
#define se second
Int mod = 1e9+7;
Int INF = 1e18;
double EPS = 0.00000001;
//Int mod = 1e6+3;

int main() {
    Int n, m;
    cin >> n >> m;
    vector<map<Int, Int>> v(m);
    for (Int i = 0; i < n; i++) {
        Int x;
        cin >> x;
        v[x % m][x]++;
    }
    vector<Int> vsum(m, 0);
    for (Int i = 0; i < m; i++) {
        for (auto j : v[i]) {
            vsum[i] += j.se;
        }
    }
    Int res = vsum[0] / 2;
    for (Int i = 1; i <= m - i; i++) {
        //cout << i << " " << min(vsum[i], vsum[m - i]) << endl;
        if (i == m - i) {
            res += vsum[i] / 2;
        }
        else {
            Int add = min(vsum[i], vsum[m - i]);
            res += add;
            if (add == 0) continue;
            Int cnt = 0;
            if (vsum[i] == add) {
                for (auto j : v[m - i]) {
                    if (cnt >= add) {
                        res += j.se / 2;
                    }
                    else if (cnt + j.se <= add) {
                        cnt += j.se;
                    }
                    else {
                        res += (add - cnt) / 2;
                        cnt += j.se;
                    }
                }
            }
            else {
                for (auto j : v[i]) {
                    if (cnt >= add) {
                        res += j.se / 2;
                    }
                    else if (cnt + j.se <= add) {
                        cnt += j.se;
                    }
                    else {
                        res += (add - cnt) / 2;
                        cnt += j.se;
                    }
                }
            }
        }
    }
    dump(res);
    return 0;
}

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