Language: C++

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <queue>
#include <functional>
#include <set>
#include <map>
#include <tuple>
#include <bitset>
#include <random>
#define REP(i,n) for(int i=0;i<n;i++)
#define INF 1000000007
using namespace std;
using pii=pair<int,int>;
using LL=int64_t;





int main(){
    int n,k;
    cin>>n>>k;
    vector<pair<int,int>> a(n);
    vector<int> neg, pos;
    int pos_cnt=0,neg_cnt=0;
    LL ans=1;

    REP(i,n){
        int b;
        cin>>b;
        if(b>=0){
            a.at(i)=make_pair(b,1);
            pos.push_back(b);
            pos_cnt++;
        }else{
            a.at(i)=make_pair(-b,-1);
            neg.push_back(-b);
            neg_cnt++;
        }
    }
    
    if(k==n){
        
        REP(i,n){
            ans*=a.at(i).first*a.at(i).second;
            ans%=INF;
        }
        if(ans<0){
            ans+=INF;
            ans%=INF;
        }
        
        cout<<ans<<endl;
        return 0;
    }


    if(neg_cnt==n&&k%2==1){
        sort(a.begin(), a.end());
        
        REP(i,k){
            ans*=a.at(i).first;
            ans%=INF;
        }
        ans*=-1;
        ans%=INF;
        ans+=INF;
        ans%=INF;
        cout<<ans<<endl;
        return 0;
    }

    sort(a.begin(), a.end(), greater<pair<int,int>>());
    sort(pos.begin(), pos.end(), greater<int>());
    sort(neg.begin(), neg.end(), greater<int>());
    
    int pos_number=0, neg_number=0;

    
    REP(i,k){
        
        if(a.at(i).second>0){
            pos_number++;
        }else{
            neg_number++;
        }
    }

    if(neg_number%2==0){
        REP(i,k){
            ans*=a.at(i).first;
            ans%=INF;
        }
        cout<<ans<<endl;
        return 0;
    }

    //本番
    int i=pos_number,j=neg_number;//i+j=k<n,j:odd>0

    if(i==0){
        REP(k,i+1){
            ans*=pos[k];
            ans%=INF;
        }
        REP(k,j-1){
            ans*=neg[k];
            ans%=INF;
        }
        cout<<ans<<endl;
        return 0;
    }

    //i>0,j>0,i+j=k<n, j:odd
    if(i<pos_cnt&&j<neg_cnt){
        LL x=pos[i]*pos[i-1],y=neg[j]*neg[j-1];
    if(y>=x){
        REP(k,i-1){
            ans*=pos[k];
            ans%=INF;
        }
        REP(k,j+1){
            ans*=neg[k];
            ans%=INF;
        }
        cout<<ans<<endl;
        return 0;
    }else{
        REP(k,i+1){
            ans*=pos[k];
            ans%=INF;
        }
        REP(k,j-1){
            ans*=neg[k];
            ans%=INF;
        }
        cout<<ans<<endl;
        return 0;
    }
    }
    
    /*if(j==neg_cnt){
        REP(k,i+1){
            ans*=pos[k];
            ans%=INF;
        }
        REP(k,j-1){
            ans*=neg[k];
            ans%=INF;
        }
        cout<<ans<<endl;
        return 0;
    }*/

    /*REP(k,i-1){
            ans*=pos[k];
            ans%=INF;
        }
        REP(k,j+1){
            ans*=neg[k];
            ans%=INF;
        }
        cout<<ans<<endl;*/


    int *a=0;
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:170:10: error: conflicting declaration 'int* a'
  170 |     int *a=0;
      |          ^
a.cc:27:27: note: previous declaration as 'std::vector<std::pair<int, int> > a'
   27 |     vector<pair<int,int>> a(n);
      |                           ^