Submission #1692741

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Source Code Expand

#include <iostream>
#include <map>
#include <vector>
using namespace std;

int main() {
    int N, M;
    cin >> N >> M;

    vector<int> p_cnt(M, 0),
                o_cnt(M, 0);
    map<int, int> mp;
    for (int i = 0; i < N; i++) {
        int X;
        cin >> X;
        mp[X] ^= 1;
        if (mp[X] == 0) {
            p_cnt[X % M]++;
        }
    }
    for (auto p : mp) o_cnt[p.first % M] += p.second;

    int ans = 0;
    for (int i1 = 0; 2 * i1 <= M; i1++) {
        int i2 = (M - i1) % M;
        if (i1 == i2) {
            ans += (2 * p_cnt[i1] + o_cnt[i1]) / 2;
        } else {
            int num1 = 2 * p_cnt[i1] + o_cnt[i1],
                num2 = 2 * p_cnt[i2] + o_cnt[i2];
            if (num1 < num2) {
                swap(num1, num2);
                swap(p_cnt[i1], p_cnt[i2]);
                swap(o_cnt[i1], o_cnt[i2]);
            }
            ans += min(num1, num2);
            ans += min(p_cnt[i1], (num1 - num2) / 2);
        }
    }
    cout << ans << endl;

    return 0;
}

Submission Info

Judge Result