Submission #1372078

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Source Code Expand

#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
using namespace std;

int main(){
	int n,m,x[100000],ans = 0,p[100000] = {},cnt[100000] = {};
	map<int,int> mp;
	cin >> n >> m;
	for(int i = 0;i < n;i++) {
		cin >> x[i];
		mp[x[i]]++;
		cnt[x[i] % m]++;
	}
	for(auto it = mp.begin();it != mp.end();it++){
		p[it->first % m] += it->second / 2;
	}
	ans += cnt[0] / 2;
	for(int i = 1;i < (m + 1) / 2;i++){
		int pa = min(cnt[i],cnt[m - i]);
		cnt[i] -= pa;
		cnt[m - i] -= pa;
		ans += pa + min(cnt[i] / 2,p[i]) + min(cnt[m - i] / 2,p[m - i]);
	}
	if(m % 2 == 0) ans += cnt[m / 2] / 2;
	cout << ans << endl;
	return 0;
}

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