Submission #992232

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Source Code Expand

#include <bits/stdc++.h>

using namespace std;

typedef long double ld;
typedef long long ll;

const int M = 100100;

int n, m;
vector<int> a[M];

void read() {
  cin >> n >> m;
  for (int i = 0; i < n; ++i) {
    int x;
    cin >> x;
    a[x % m].push_back(x);
  }
  for (int i = 0; i < m; ++i)
    sort(a[i].begin(), a[i].end());
}

int cnt(vector<int> x) {
  map<int, int> c;
  for (int a : x)
    c[a]++;
  int ans = 0;
  for (auto p : c)
    ans += p.second / 2;
  return ans;
}

void kill() {
  int ans = 0;
  for (int i = 0; i <= m - i; ++i) {
    int j = (m - i) % m;
    if (i == j) {
      ans += a[i].size() / 2;
    } else {
      auto x = a[i];
      auto y = a[j];

      if (x.size() > y.size())
        swap(x, y);

      int xs = x.size();
      int ys = y.size();

      int t = cnt(y);

      ans += xs + min(t, (ys - xs) / 2);
    }
  }
  cout << ans << endl;
}

int main() {
#ifdef LOCAL
  assert(freopen("d.in", "r", stdin));
#endif

  ios_base::sync_with_stdio(false);

  read();
  kill();

}

Submission Info

Judge Result