Submission #992245

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Source Code Expand

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;

#define EACH(i,a) for (auto& i : a)
#define FOR(i,a,b) for (ll i=(a);i<(b);i++)
#define RFOR(i,a,b) for (ll i=(b)-1;i>=(a);i--)
#define REP(i,n) for (ll i=0;i<(n);i++)
#define RREP(i,n) for (ll i=(n)-1;i>=0;i--)
#define debug(x) cout<<#x<<": "<<x<<endl
#define pb push_back
#define ALL(a) (a).begin(),(a).end()

const ll linf = 1e18;
const int inf = 1e9;
const double eps = 1e-12;
const double pi = acos(-1);

template<typename T>
istream& operator>>(istream& is, vector<T>& vec) {
    EACH(x,vec) is >> x;
    return is;
}
template<typename T>
ostream& operator<<(ostream& os, vector<T>& vec) {
    REP(i,vec.size()) {
        if (i) os << " ";
        os << vec[i];
    }
    return os;
}
template<typename T>
ostream& operator<<(ostream& os, vector< vector<T> >& vec) {
    REP(i,vec.size()) {
        if (i) os << endl;
        os << vec[i];
    }
    return os;
}

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    int n, m; cin >> n >> m;
    vector<int> X(n); cin >> X;
    map<int, int> cnt, cnt2;
    REP(i, n) {
        cnt[X[i]%m]++;
        cnt2[X[i]]++;
    }
    map<int, int> self;
    EACH(p, cnt2) {
        self[p.first%m] += p.second/2;
    }
    set<int> used;
    ll ans = 0;
    EACH(p, cnt) {
        int x, c; tie(x, c) = p;
        if ( used.count(x) > 0 ) continue;
        int y = m-x;
        used.insert(x);
        used.insert(y);
        if (x == y) {
            ans += c/2;
        }
        else {
            int c1 = c, c2 = cnt[y];
            int l1 = cnt2[x], l2 = cnt2[y];
            int M = 0;
            REP(i, min(c1, c2)+1) {
                int a = i;
                a += min<int>((c1-i)/2, l1);
                a += min<int>((c2-i)/2, l2);
                M = max(M, a);
            }
            ans += M;
        }
    }
    cout << ans << endl;
}

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