Submission #994882

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;

#define fi first
#define se second
#define repl(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repl(i,0,n)
#define each(itr,v) for(auto itr:v)
#define pb(s) push_back(s)
#define mp(a,b) make_pair(a,b)
#define all(x) (x).begin(),(x).end()
#define dbg(x) cout<<#x"="<<x<<endl
#define maxch(x,y) x=max(x,y)
#define minch(x,y) x=min(x,y)
#define uni(x) x.erase(unique(all(x)),x.end())
#define exist(x,y) (find(all(x),y)!=x.end())
#define bcnt(x) bitset<32>(x).count()

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef pair<P, int> PPI;
typedef pair<ll, ll> PL;
typedef pair<P, ll> PPL;

#define INF INT_MAX/3

#define MAX_N 1000

ll n,m;
ll cnt[111111];
vector<ll> mod[111111];
bool used[111111];

int main(){
	cin.sync_with_stdio(false);
	cin>>n>>m;
	rep(i,n){
		ll x;
		cin>>x;
		mod[x%m].pb(x);
		cnt[x]++;
	}
	rep(i,m){
		sort(all(mod[i]));
	}
	ll res=mod[0].size()/2;
	repl(i,1,m/2+1){
		ll add;
		if(i!=m-i)add=min(mod[i].size(),mod[m-i].size());
		else{
			res+=mod[i].size()/2;
			continue;
		}
		res+=add;
		ll pnum=0,bnum=0;
		rep(j,mod[i].size()){
			if(used[mod[i][j]])continue;
			used[mod[i][j]]=true;
			pnum+=cnt[mod[i][j]]/2;
			bnum+=cnt[mod[i][j]]%2;
		}
		if(add-bnum>0){
			res+=max(0LL,(pnum*2-(add-bnum))/2);
		}else{
			res+=pnum;
		}
		if(i!=m-i){
			pnum=0,bnum=0;
			rep(j,mod[m-i].size()){
				if(used[mod[m-i][j]])continue;
				used[mod[m-i][j]]=true;
				pnum+=cnt[mod[m-i][j]]/2;
				bnum+=cnt[mod[m-i][j]]%2;
			}
			if(add-bnum>0){
				res+=max(0LL,(pnum*2-(add-bnum))/2);
			}else{
				res+=pnum;
			}
		}
	}
	cout<<res<<endl;
	return 0;
}

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