Language: C++

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

#define rep(i,n) for(ll i=0;i<(n);++i)
using ll = int_fast64_t;
using pll = pair<ll,ll>;
constexpr ll INF = 1LL<<60;
constexpr ll MOD = 1e9+7;
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}return 0;}
#if loc||local
template<class T>void dump(T&& t){cerr<<t<<endl;}
template<class T,class... Ts> void dump(T&& h, Ts&&... t){cerr<<h<<", ";dump(forward<Ts>(t)...);}
#else
void dump(){}
template<class T,class... Ts> void dump(T&& h, Ts&&... t){}
#endif
template<class T> istream &operator>>(istream&is,vector<T>&v){for(auto &elemnt:v)is>>elemnt;return is;}
template<class T,class U> istream &operator>>(istream&is,pair<T,U>&p){is>>p.first>>p.second;return is;}
template<class T> ostream &operator<<(ostream& os,vector<T>const& v){for(auto const& vi:v)os<<vi<<" ";return os;}
template<class T,class U> ostream &operator<<(ostream& os,pair<T,U>const& p){os<<p.first<<","<<p.second;return os;}
template<class T>vector<T> vec(size_t a){return vector<T>(a);}
template<class T, class... Ts>auto vec(size_t a, Ts... ts){return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...));}
template<int64_t MOD>
struct mod_int{
  int64_t val;
  constexpr mod_int():val(0){}
  constexpr mod_int(int64_t x)noexcept:val(x>=0?x%MOD:(MOD-(-x)%MOD)%MOD){}
  constexpr int64_t value()const noexcept{return val;}
  constexpr int64_t get_MOD()const noexcept{return MOD;}
  constexpr mod_int operator+(mod_int const& rhs){
    return mod_int(*this)+=rhs;
  }
  constexpr mod_int operator-(mod_int const& rhs){
    return mod_int(*this)-=rhs;
  }
  constexpr mod_int operator*(mod_int const& rhs){
    return mod_int(*this)*=rhs;
  }
  constexpr mod_int operator/(mod_int const& rhs){
    return mod_int(*this)/=rhs;
  }
  constexpr mod_int &operator+=(mod_int const& rhs)noexcept{
    val += rhs.val;
    if(val>=MOD)val-=MOD;
    return *this;
  }
  constexpr mod_int &operator-=(mod_int const& rhs)noexcept{
    if(val<rhs.val)val+=MOD;
    val-=rhs.val;
    return *this;
  }
  constexpr mod_int &operator*=(mod_int const& rhs)noexcept{
    (val*=rhs.val)%=MOD;
    return *this;
  }
  constexpr mod_int &operator/=(mod_int rhs)noexcept{
    *this*=rhs.inverse();
    return *this;
  }
  constexpr bool operator==(mod_int const& rhs)const{
    return val==rhs.val;
  }
  constexpr bool operator!=(mod_int const& rhs)const{
    return !(val==rhs.val);
  }
  constexpr bool operator<(mod_int const& rhs)const{
    return val<rhs.val;
  }
  constexpr bool operator>(mod_int const& rhs)const{
    return val>rhs.val;
  }
  constexpr bool operator<=(mod_int const& rhs)const{
    return !(val>rhs.val);
  }
  constexpr bool operator>=(mod_int const& rhs)const{
    return !(val<rhs.val);
  }
  constexpr friend std::ostream &operator<<(std::ostream& os, mod_int const& mi){
    return os<<mi.val;
  }
  constexpr friend std::istream &operator>>(std::istream& is, mod_int & mi){
    int64_t t=0;is>>t;
    mi = mod_int<MOD>(t);
    return is;
  }
  constexpr mod_int inverse(){
    int64_t a=val,b=MOD,u=1,v=0,t=0;
    while(b>0){
      t=a/b;
      std::swap(a-=t*b,b);
      std::swap(u-=t*v,v);
    }
    return mod_int(u);
  }
  constexpr mod_int mpow(int64_t n)const{
    mod_int res(1),e(*this);
    while(n){
      if(n&1)res*=e;
      e*=e;
      n>>=1;
    }
    return res;
  }
};
using mint = mod_int<1000000007>;
mint mpow(int64_t p,int64_t n){
  mint res=1;
  mint e = p;
  while(n){
    if(n&1)res*=e;
    e*=e;
    n>>=1;
  }
  return res;
}

struct combination{
  using i64 = int64_t;
  static constexpr i64 COM_MAX = 2e6;
  static constexpr i64 MOD = 1e9+7;
  vector<i64> fac,inv,finv;
  combination(){
    fac.resize(COM_MAX);
    inv.resize(COM_MAX);
    finv.resize(COM_MAX);
    fac[0] = fac[1] = 1;
    inv[1] = 1;
    finv[0] = finv[1] = 1;
    for(int i=2;i<COM_MAX;++i){
      fac[i]=fac[i-1]*i%MOD;
      inv[i]=MOD-inv[MOD%i]*(MOD/i)%MOD;
      finv[i]=finv[i-1]*inv[i]%MOD;
    }
  }
  template<class T>
  mint nCk(T n,T k){
    if(n<k)return 0;
    if(n<0||k<0)return 0;
    if(n>=COM_MAX){
      mint res1 = mint(1);
      mint res2 = mint(fac[k]);
      for(int i=1;i<=k;++i){
        res1 *= n-(i-1);
      }
      return res1/res2;
    }
    return mint(fac[n]*(finv[k]*finv[n-k]%MOD));
  }
  template<class T>
  mint nHk(T n,T k){
    return nCk(n+k-1,k-1);
  }
  template<class T>
  mint operator()(T n,T k){
    return nCk(n,k);
  }
};

signed main(){

  int n,k;
  cin>>n>>k;
  vector<ll> a(n);
  cin>>a;

  sort(begin(a),end(a),[](auto a,auto b){return abs(a)>abs(b);});
  mint ans = 1;
  int t = 0;
  rep(i,k){
    ans *= abs(a[i]);
    if(a[i]<0)t^=1;
  }
  if(t==0){
    cout<< ans <<endl;
    return 0;
  }
  bool fa,fb,fc,fd;
  fa = fb = fc = fd = false;
  ll aa,bb,cc,dd;
  for(int i=k;i<n;++i)if(a[i]>0){
    aa = a[i];
    fa = true;
    break;
  }
  for(int i=k;i<n;++i)if(a[i]<0){
    bb = a[i];
    fb = true;
    break;
  }
  for(int i=k;i-->0;)if(a[i]>0){
    cc = a[i];
    fc = true;
    break;
  }
  for(int i=k;i-->0;)if(a[i]<0){
    dd = a[i];
    fd = true;
    break;
  }
  // a/d b/c <-> ac bd
  if(fa&&fb&&fc&&fd){
    if(a*c>b*d){
      ans /= d;
      ans *= a;
    }else{
      ans /= c;
      ans *= b;
    }
  }
  else if(fa&&fd){
    ans /= d;
    ans *= a;
  }
  else if(fb&&fc){
    ans /= c;
    ans *= b;
  }
  else{
    sort(begin(a),end(a),[](auto a,auto b){return abs(a)<abs(b);});
    ans = 1;
    rep(i,k)ans*=a[i];
  }
  cout<<(ans)<<endl;

}

Compiler output:
a.cc: In function 'int main()':
a.cc:203:10: error: 'c' was not declared in this scope; did you mean 'cc'?
  203 |     if(a*c>b*d){
      |          ^
      |          cc
a.cc:203:12: error: 'b' was not declared in this scope; did you mean 'bb'?
  203 |     if(a*c>b*d){
      |            ^
      |            bb
a.cc:203:14: error: 'd' was not declared in this scope; did you mean 'dd'?
  203 |     if(a*c>b*d){
      |              ^
      |              dd
a.cc:205:11: error: no match for 'operator*=' (operand types are 'mint' {aka 'mod_int<1000000007>'} and 'std::vector<long int>')
  205 |       ans *= a;
      |       ~~~~^~~~
a.cc:53:22: note: candidate: 'constexpr mod_int<MOD>& mod_int<MOD>::operator*=(const mod_int<MOD>&) [with long int MOD = 1000000007]'
   53 |   constexpr mod_int &operator*=(mod_int const& rhs)noexcept{
      |                      ^~~~~~~~
a.cc:53:48: note:   no known conversion for argument 1 from 'std::vector<long int>' to 'const mod_int<1000000007>&'
   53 |   constexpr mod_int &operator*=(mod_int const& rhs)noexcept{
      |                                 ~~~~~~~~~~~~~~~^~~
a.cc:212:12: error: 'd' was not declared in this scope; did you mean 'dd'?
  212 |     ans /= d;
      |            ^
      |            dd
a.cc:213:9: error: no match for 'operator*=' (operand types are 'mint' {aka 'mod_int<1000000007>'} and 'std::vector<long int>')
  213 |     ans *= a;
      |     ~~~~^~~~
a.cc:53:22: note: candidate: 'constexpr mod_int<MOD>& mod_int<MOD>::operator*=(const mod_int<MOD>&) [with long int MOD = 1000000007]'
   53 |   constexpr mod_int &operator*=(mod_int const& rhs)noexcept{
      |                      ^~~~~~~~
a.cc:53:48: note:   no known conversion for argument 1 from 'std::vector<long int>' to 'const mod_int<1000000007>&'
   53 |   constexpr mod_int &operator*=(mod_int const& rhs)noexcept{
      |                                 ~~~~~~~~~~~~~~~^~~
a.cc:216:12: error: 'c' was not declared in this scope; did you mean 'cc'?
  216 |     ans /= c;
      |            ^
      |            cc
a.cc:217:12: error: 'b' was not declared in this scope; did you mean 'bb'?
  217 |     ans *= b;
      |            ^
      |            bb