Codeforces Round #544 (Div. 3) B. Preparation for International Women's Day题解

https://codeforces.com/contest/1133/problem/B

#include<algorithm>
#include <iostream>
#include  <sstream>
#include  <cstring>
#include  <cstdlib>
#include   <string>
#include   <vector>
#include   <cstdio>
#include   <math.h>
#include    <queue>
#include    <stack>
#include      <set>
typedef long long ll;
typedef unsigned long long ull;
#define rep(i,a,n) for (int i = a; i < n; ++i)
#define per(i,a,n) for (int i = n-1; i >= a; --i)
#define SZ(x) ((int)x.size())
using namespace std;
//head

#define maxn 1000006
int a[maxn] = { 0 }, cnt[maxn] = { 0 };
int main()
{
	int n, k;
	cin >> n >> k;
	for (int i = 0; i < n; i++)
	{
		cin >> a[i];
		cnt[a[i] % k] ++;
	}
	int ans = (cnt[0] / 2);//本来就等于k的两两相组
	for (int i = 1; i * 2 <= k; i++)
	{
		int x = min(cnt[i], cnt[k - i]);
		if (i * 2 == k)
			ans += x / 2;//去重，与初始化同理
		else
			ans += x;
	}
	cout << ans * 2 << endl;
	//system("pause");
	return 0;
}