Submission #7507473

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Source Code Expand

#include <bits/stdc++.h>
#define loop(n) for (int ngtkana_is_genius = 0; ngtkana_is_genius < int(n); ngtkana_is_genius++)
#define rep(i, begin, end) for(int i = int(begin); i < int(end); i++)
#define all(v) v.begin(), v.end()
#define lint long long
auto cmn = [](auto& a, auto b){if (a > b) {a = b; return true;} return false;};
auto cmx = [](auto& a, auto b){if (a < b) {a = b; return true;} return false;};
void debug_impl() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_impl(Head head, Tail... tail){
  std::cerr << " " << head;
  debug_impl(tail...);
}
#define debug(...)\
  std::cerr << std::boolalpha << "[" << #__VA_ARGS__ << "]:";\
  debug_impl(__VA_ARGS__);\
  std::cerr << std::noboolalpha;

template <typename T>
std::istream& operator>> (std::istream& is, std::vector<T>& v) {
  for (auto & x : v) is >> x;
  return is;
}

template <typename T>
std::ostream& operator<< (std::ostream& os, const std::vector<T>& v) {
  auto n = v.size();
  os << "{";
  for (size_t i = 0; i < n; i++)
    {os << (i > 0 ? "," : "") << v.at(i);}
  return os << "}";
}

template <typename T, typename U>
std::ostream& operator<< (std::ostream& os, const std::pair<T, U>& pair)
  { return os << "(" << pair.first << "," << pair.second << ")"; }

template <typename T, typename U>
std::istream& operator>> (std::iostream& is, std::pair<T, U>& pair)
  { return is >> pair.first >> pair.second; }

template <typename T>
auto make_vector_impl(size_t sz, T t) {return std::vector<T>(sz, t);}

template <size_t N, typename T, typename U, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz, U u) {return make_vector_impl(sz, T(u));}

template <size_t N, typename T, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz) {return std::vector<T>(sz);}

template <size_t N, typename T, typename... Args, std::enable_if_t<N != 1, std::nullptr_t> = nullptr>
auto make_vector(size_t a, Args... args) {return make_vector_impl(a, make_vector<N - 1, T>(args...));}

template <typename T, typename Size_t>
auto& at(T& t, Size_t i) {return t.at(i);}

template <typename T, typename Size_t, typename... Args>
auto& at(T& t, Size_t i, Args... args) {return at(t.at(i), args...);}

int main() {
  std::cin.tie(0); std::cin.sync_with_stdio(false);
  int n, m; std::cin >> n >> m;
  auto a = make_vector< 2, int >(m, 0);
  loop(n){
    int x; std::cin >> x;
    a.at(x % m).emplace_back(x);
  }
  for(auto & v : a) std::sort(all(v));
  auto b = std::vector< std::pair< int, int >>(m); // cnt, pairing
  rep(i, 0, m) {
    auto prv = -1, now = 0, pairing = 0;
    for (auto crr : a.at(i)) {
      if (prv != crr && prv != -1) {
        pairing += now / 2;
        now = 0;
      }
      now++;
      prv = crr;
    }
    pairing += now / 2;
    b.at(i) = { a.at(i).size(), pairing };
  }
  auto cal = [&] (auto&& p, auto&& q) {
    int a, b, c, d;
    std::tie(a, b) = p;
    std::tie(c, d) = q;
    if (a > c) {
      std::swap(a, c);
      std::swap(b, d);
    }
    return a + std::min(c - a, d);
  };
  auto dal = [&] (auto&& p)
    { return p.first / 2; };
  auto ret = 0LL;
  rep(i, 0, m) {
    auto j = m - i;
    if (i > j) break;
    else if (i == 0 || i == j) ret += dal(b.at(i));
    else ret += cal(b.at(i), b.at(j));
  }
  // debug(b);
  std::cout << ret << std::endl;
  return 0;
}

Submission Info

Judge Result