Submission #992433

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
#define FOR(i,k,n) for(int i = (int)(k); i < (int)(n); i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) a.begin(), a.end()
#define MS(m,v) memset(m,v,sizeof(m))
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int, int> pii;
const int MOD = 1000000007;
const int INF = MOD + 1;
const ld EPS = 1e-12;
template<class T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template<class T> T &chmax(T &a, const T &b) { return a = max(a, b); }

/*--------------------template--------------------*/

int main()
{
	cin.sync_with_stdio(false); cout << fixed << setprecision(10);
	int n, m; cin >> n >> m;
	vi x(n);
	REP(i, n) cin >> x[i];
	sort(ALL(x));
	vector<vi> v(m);
	for(auto i : x)
	{
		v[i % m].push_back(i);
	}
	vi p(m);
	REP(i, m)
	{
		REP(j, v[i].size() - 1)
		{
			if (v[i][j] == v[i][j + 1])
			{
				p[i]++;
				j++;
			}
		}
	}
	int ans = 0;
	ans += v[0].size() / 2;
	if (m % 2 == 0) ans += v[m / 2].size() / 2;
	FOR(i, 1, (m+1) / 2)
	{
		int j = m - i;
		int a = v[i].size(), b = v[j].size();
		if (a >= b)
		{
			ans += b;
			int np = a - 2 * p[i];
			if (np >= b) ans += p[i];
			else ans += (a - b) / 2;
		}
		else
		{
			ans += a;
			int np = b - 2 * p[j];
			if (np >= a) ans += p[j];
			else ans += (b - a) / 2;
		}
	}
	cout << ans << endl;
	return 0;
}

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