Submission #992240

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Source Code Expand

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define fbo find_by_order
#define ook order_of_key

typedef long long ll;
typedef pair<ll,ll> ii;
typedef vector<int> vi;
typedef long double ld; 
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;
typedef set<int>::iterator sit;
typedef map<int,int>::iterator mit;
typedef vector<int>::iterator vit;

int n,m;
map<int,int> cnt[100001];
vi vec[100001];

int main()
{
	ios_base::sync_with_stdio(0); cin.tie(0);
	cin>>n>>m;
	for(int i=0;i<n;i++)
	{
		int x; cin >> x;
		cnt[x%m][x]++;
	}
	for(int i = 0; i < m; i++)
	{
		for(mit it = cnt[i].begin(); it != cnt[i].end(); it++)
		{
			vec[i].pb(it->se);
		}
	}
	int ans = 0;
	for(int i = 0; 2*i <= m; i++)
	{
		if(i==0||i==m-i)
		{
			int sum=0;
			for(int j = 0; j < vec[i].size(); j++)
			{
				sum+=vec[i][j];
			}
			ans+=sum/2;
		}
		else
		{
			int clr1 = 0;
			int clr2 = 0;
			int sum1 = 0;
			int sum2 = 0;
			for(int j = 0; j < vec[i].size(); j++)
			{
				sum1 += vec[i][j];
				clr1 += (vec[i][j]/2);
			}
			for(int j = 0; j < vec[m-i].size(); j++)
			{
				sum2 += vec[m-i][j];
				clr2 += (vec[m-i][j]/2);
			}
			if(sum1>=sum2)
			{
				int mindel = min(clr1, (sum1-sum2)/2);
				sum1 -= 2*mindel;
				ans += min(sum1,sum2) + mindel;
			}
			else
			{
				int mindel = min(clr2, (sum2-sum1)/2);
				sum2 -= 2*mindel;
				ans += min(sum1,sum2) + mindel;
			}
		}
	}
	cout<<ans<<'\n';
}

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