Submission #3222594

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;
template<class T> void assign(V<T>& v, int n, const auto& a = T()) { v.assign(n, a); }
template<class T, class... U> void assign(V<T>& v, int n, const U&... u) { v.resize(n); for (auto&& i : v) assign(i, u...); }

const int max_n = 1e5;
int main() {
  cin.tie(NULL); ios::sync_with_stdio(false);
  int n, m; cin >> n >> m;
  V<> c(max_n + 1);
  for (int i = 0; i < n; i++) {
    int x; cin >> x;
    c[x]++;
  }
  V<> r(m), p(m);
  for (int i = 1; i < max_n + 1; i++) {
    r[i % m] += c[i];
    p[i % m] += c[i] / 2;
  }
  int res = r[0] / 2;
  for (int i = 1; i < (m + 1) / 2; i++) {
    if (r[i] > r[m - i]) res += r[m - i] + min((r[i] - r[m - i]) / 2, p[i]);
    else res += r[i] + min((r[m - i], r[i]) / 2, p[m - i]);
  }
  if (!(m & 1)) res += r[m / 2] / 2;
  cout << res << '\n';
}

Submission Info

Judge Result