Submission #7901261

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Source Code Expand

// #define DEBUGGING
#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define ALL(V) (V).begin(), (V).end()
#define ALLR(V) (V).rbegin(), (V).rend()

template <typename T> using V = vector<T>;
template <typename T> using VV = V<V<T>>;
using ll = int64_t;
using ull = uint64_t;
using PLL = pair<ll, ll>;

template <typename T> const T& var_min(const T &t) { return t; }
template <typename T> const T& var_max(const T &t) { return t; }
template <typename T, typename... Tail> const T& var_min(const T &t, const Tail&... tail) { return min(t, var_min(tail...)); }
template <typename T, typename... Tail> const T& var_max(const T &t, const Tail&... tail) { return max(t, var_max(tail...)); }
template <typename T, typename... Tail> void chmin(T &t, const Tail&... tail) { t = var_min(t, tail...); }
template <typename T, typename... Tail> void chmax(T &t, const Tail&... tail) { t = var_max(t, tail...); }
template <typename T> const T& clamp(const T &t, const T &low, const T &high) { return max(low, min(high, t)); }
template <typename T> void chclamp(T &t, const T &low, const T &high) { t = clamp(t, low, high); }

namespace init__ {

struct InitIO {
    InitIO() {
        cin.tie(nullptr);
        ios_base::sync_with_stdio(false);
        cout << fixed << setprecision(30);
    }
} init_io;

}

#ifdef DEBUGGING
// #include "../debug/debug.cpp"
#include "../../debug/debug.cpp"
#else
#define DEBUG(...) 0
#define DEBUG_SEPARATOR_LINE 0
#endif

template <typename T>
T make_v(T init) { return init; }

template <typename T, typename... Tail>
auto make_v(T init, size_t s, Tail... tail) {
#define rec make_v(init, tail...)
    return V<decltype(rec)>(s, rec);
#undef rec
}

int main() {
    ll N, M;
    cin >> N >> M;
    VV<ll> modm(M);
    V<ll> cnt(M);
    for(ll i = 0; i < N; i++) {
        ll X;
        cin >> X;
        modm[X % M].push_back(X);
    }
    
    for(ll i = 0; i < M; i++) {
        sort(ALL(modm[i]));
        ll pre = -1;
        ll cnt_e = 0;
        for(ll e : modm[i]) {
            if(pre == e) {
                cnt_e++;
            } else {
                cnt[i] += cnt_e / 2;
                cnt_e = 0;
            }
            pre = e;
        }
        cnt[i] += cnt_e / 2;
    }

    ll ans = modm[0].size() / 2;
    DEBUG(modm);
    for(ll mod = 1; mod < M / 2 + (M & 1); mod++) {
        ll i = mod, j = M - mod;
        if(modm[i].size() < modm[j].size()) swap(i, j);
        ans += modm[j].size();
        ll diff = modm[i].size() - modm[j].size();
        ans += min(cnt[i], diff / 2);
        DEBUG(ans);
    }
    if(M % 2 == 0) ans += modm[M / 2].size() / 2;
    cout << ans << endl;
    return 0;
}

Submission Info

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