Submission #7939429

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Source Code Expand

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

#define rep(i, j) for (int i = 0; i < j; i++)
#define all(obj) (obj).begin(), (obj).end()
#define rall(obj) (obj).rbegin(), (obj).rend()
typedef long long int ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<pii> vpii;

int main() {

    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.precision(10);
    cout << fixed;

    int n, m;
    cin >> n >> m;

    map<int, int> mp;
    rep(i, n) {
        int x;
        cin >> x;
        mp[x]++;
    }

    map<int, int> mpe, mpo;
    for (auto& p: mp) {
        mpe[p.first % m] += p.second - p.second % 2;
        if (p.second % 2 > 0) mpo[p.first % m]++;
    }

    ll ans = mpe[0] / 2;
    if (m % 2 == 0) {
        ans += mpe[m / 2] / 2;
    }

    for (int i = 1; i <= (m - 1) / 2; i++) {
        if (mpo[i] > mpo[m - i]) {
            int x = min(mpe[m - i], mpo[i] - mpo[m - i]);
            ans += mpo[m - i] + x;
            mpe[m - i] -= x;
        } else {
            int x = min(mpe[i], mpo[m - i] - mpo[i]);
            ans += mpo[i] + x;
            mpe[i] -= x;
        }
    }

    // for (int i = 1; i <= (m - 1) / 2; i++) {  
    rep(i, m) {
        // cout << i << " " << mpe[i] << endl;
        ans += mpe[i] / 2;
    }

    cout << ans << endl;
    return 0;
}

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