Submission #5890297

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Source Code Expand

#include <bits/stdc++.h>
#define GET_MACRO(_1,_2,_3,_4,_5,_6,_7,_8,NAME,...) NAME
#define pr(...) cerr<< GET_MACRO(__VA_ARGS__,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__) <<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define prArr(a) {cerr<<(#a)<<"={";int i=0;for(auto t:(a))cerr<<(i++?", ":"")<<t;cerr<<"}"<<endl;}
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){
  return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE


signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  int N, M;
  cin>>N>>M;
  vector<int> X(N);
  cin>>X;

  map<int, int> cnt[M];
  for(int x:X) cnt[x%M][x]++;

  auto calc = [&](int a, int b){
    if(a == b){
      int sum = 0;
      for(auto p:cnt[a]) sum += p.second;
      return sum / 2;
    }
    
    int res = 0;    
    deque<P> A, B;
    for(auto p:cnt[a]) A.emplace_back(p.second, p.first);
    for(auto p:cnt[b]) B.emplace_back(p.second, p.first);
    sort(A.begin(), A.end(), [](auto &p, auto &q){
	return p.first%2 > q.first%2;
      });
    
    sort(B.begin(), B.end(), [](auto &p, auto &q){
	return p.first%2 > q.first%2;
      });
    
    while(1){
      if(A[0].first == 0) A.pop_front();
      if(B[0].first == 0) B.pop_front();
      if(A.empty() || B.empty()) break;
      if(A[0].first % 2 == 0 || B[0].first % 2 == 0) break;
      res++;
      A[0].first--;
      B[0].first--;
      A.push_back(A[0]), A.pop_front();
      B.push_back(B[0]), B.pop_front();
    }
    
    while(!A.empty()) res += A[0].first / 2, A.pop_front();
    while(!B.empty()) res += B[0].first / 2, B.pop_front();
    return res;
  };

  int ans = 0;
  for(int i=0;i<M;i++){
    int a = i;
    int b = (M - i) % M;
    if(a <= b) ans += calc(a, b);
  }

  cout<<ans<<endl;
  
  return 0;
}

Submission Info

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