Language: C++

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

typedef long long ll;
typedef vector<ll> vint;
typedef vector< vector<ll> > vvint;

#define rep(i,n) for(ll i=0;i<n;i++)
#define repf(i,f,n) for(ll i=f;i<n;i++)
#define repr(i,n) for(ll i=n-1;i>=0;i--)
#define mp make_pair
#define mt make_tuple
#define ALL(obj) (obj).begin(), (obj).end()
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 (b<a) { a=b; return 1; } return 0; }

int dy[]={0, 0, 1, -1, 1, -1, -1, 1};
int dx[]={1, -1, 0, 0, 1, 1, -1, -1};

template< int mod >
struct ModInt {
int x;

ModInt() : x(0) {}

ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

ModInt &operator+=(const ModInt &p) {
if((x += p.x) >= mod) x -= mod;
return *this;
}

ModInt &operator-=(const ModInt &p) {
if((x += mod - p.x) >= mod) x -= mod;
return *this;
}

ModInt &operator*=(const ModInt &p) {
x = (int) (1LL * x * p.x % mod);
return *this;
}

ModInt &operator/=(const ModInt &p) {
*this *= p.inverse();
return *this;
}

ModInt operator-() const { return ModInt(-x); }

ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

bool operator==(const ModInt &p) const { return x == p.x; }

bool operator!=(const ModInt &p) const { return x != p.x; }

ModInt inverse() const {
int a = x, b = mod, u = 1, v = 0, t;
while(b > 0) {
t = a / b;
swap(a -= t * b, b);
swap(u -= t * v, v);
}
return ModInt(u);
}

ModInt pow(int64_t n) const {
ModInt ret(1), mul(x);
while(n > 0) {
if(n & 1) ret *= mul;
mul *= mul;
n >>= 1;
}
return ret;
}

friend ostream &operator<<(ostream &os, const ModInt &p) {
return os << p.x;
}

friend istream &operator>>(istream &is, ModInt &a) {
int64_t t;
is >> t;
a = ModInt< mod >(t);
return (is);
}

static int get_mod() { return mod; }
};

using modint = ModInt< (int)(1e9+7) >;

template< typename T >
struct Combination {
vector< T > _fact, _rfact, _inv;

Combination(int sz) : _fact(sz + 1), _rfact(sz + 1), _inv(sz + 1) {
_fact[0] = _rfact[sz] = _inv[0] = 1;
for(int i = 1; i <= sz; i++) _fact[i] = _fact[i - 1] * i;
_rfact[sz] /= _fact[sz];
for(int i = sz - 1; i >= 0; i--) _rfact[i] = _rfact[i + 1] * (i + 1);
for(int i = 1; i <= sz; i++) _inv[i] = _rfact[i] * _fact[i - 1];
}

inline T fact(int k) const { return _fact[k]; }

inline T rfact(int k) const { return _rfact[k]; }

inline T inv(int k) const { return _inv[k]; }

T P(int n, int r) const {
if(r < 0 || n < r) return 0;
return fact(n) * rfact(n - r);
}

T C(int p, int q) const {
if(q < 0 || p < q) return 0;
return fact(p) * rfact(q) * rfact(p - q);
}

T H(int n, int r) const {
if(n < 0 || r < 0) return (0);
return r == 0 ? 1 : C(n + r - 1, r);
}
};

template<class Int> struct Fraction{//{{{
    Int num, den;

    private:
    static Int gcd(Int a, Int b){
        while(b) swap(a %= b, b);
        return a;
    }
    void normalize(){
        Int g = gcd(num, den);
        num /= g; den /= g;
        if(den < 0){ num = -num; den = -den; }
    }
    public:
    Fraction(Int num) : num(num), den(1) { }
    Fraction(Int num, Int den, bool need_normalize = true) : num(num), den(den) {
        if(need_normalize) normalize();
    }

    Fraction operator-() const {
        return Fraction(-num, den, false);
    }
};//}}}

using Z = int64_t;
using Q = Fraction<Z>;

// int sgn(const Z a){
//     return (a > 0) - (a < 0);
// }
int sgn(const Z a, const Z b){
    return (a > b) - (a < b); // sgn(a-b) にするとオーバーフローしやすい.
}
// int sgn(const Q &a){
//     return sgn(a.num);
// }
// int sgn(const Q &a, const Q &b){
//     return sgn(a.num * b.den, b.num * a.den); // オーバーフローしやすい.
// }

int sgn(Q a, Q b){
    if(a.num <= 0 or b.num <= 0){
        if(a.num == 0 or b.num == 0) return sgn(a.num, b.num);
        if((a.num < 0) xor (b.num < 0)) return sgn(a.num, b.num);
        return sgn(-b, -a);
    }
    int flip = 1;
    while(a.den != 0 and b.den != 0){
        if(int s = sgn(a.num / a.den, b.num / b.den)) return s * flip;
        swap(a.num %= a.den, a.den);
        swap(b.num %= b.den, b.den);
        flip = -flip;
    }
    return flip * sgn(a.num * b.den, a.den * b.num);
}

int main() {
    cout<<fixed<<setprecision(10);

    // auto a = Fraction<ll>(4, 5);
    // auto b = Fraction<ll>(5,6);

    // cout<<sgn(a,b)<<endl; //
    // cout<<sgn(b,a)<<endl;

    ll n,k; cin>>n>>k;
    vint a(n); rep(i,n) cin>>a[i];

    using P = pair<ll,ll>;
    vector<P> vp;
    for(auto e:a){
        if(e<0){
            vp.emplace_back(-e, 1);
        }else{
            vp.emplace_back(e, 0);
        }
    }
    sort(ALL(vp), greater<P>());

    ll posi, negi;
    posi = -1;
    negi = -1;
    ll negcnt = 0;
    rep(i,k){
        ll e,t;
        tie(e,t) = vp[i];
        if(t==1){
            negcnt++;
            negi = i;
        }else{
            posi = i;
        }
    }

    if(vp[k-1].first == 0){
        cout<<0<<endl;
        return 0;
    }

    if(negcnt%2==0){
        modint ans = 1;
        rep(i,k) ans *= vp[i].first;
        cout<<ans<<endl;
        return 0;

    }else{
        if(posi==-1){
            ll num = -1;
            repf(i,k,n){
                ll e,t;
                tie(e,t) = vp[i];
                if(t==1) continue;
                num = 1;
                swap(vp[negi], vp[i]);
                break;
            }
            if(num== -1){
                // 負の数だけで
                modint ans =1;
                rep(i,k){
                    ans *= vp[n-1-i].first;
                }
                ans *= -1;
                cout<<ans<<endl;
                return 0;

           }else{
            //    auto x = Fraction<ll>(num, vp[negi].first);
                modint ans = 1;
                rep(i,k) ans *= vp[i].first;
                cout<<ans<<endl;
                return 0;
           }
        }
        else{
            auto x = Fraction<ll>(1);
            auto y = Fraction<ll>(1);

            bool nf,pf;
            nf = false; pf = false;
            ll nposi, nnegi;
            repf(i,k,n){
                ll e,t; tie(e,t) = vp[i];
                if(t==1){
                    if(nf==false){
                        x = Fraction<ll>(e, vp[posi].first);
                        nnegi = i;
                        nf = true;
                    }
                }else{
                    if(pf==false){
                        y = Fraction<ll>(e, vp[negi].first);
                        nposi = i;
                        pf = true;
                    }
                }
                if(nf and pf) break;
            }

            modint ans = 1;


            if(sgn(x,y) < 0){
                // yがおおきい
                swap(vp[negi], vp[nposi]);
                rep(i,k){
                    ans *= vp[i].first;
                }
            }else{
                // xがおおきい
                swap(vp[posi], vp[nnegi]);
                rep(i,k){
                    ans *= vp[i].first;
                }
            }
            cout<<ans<<endl;
        }
    }





    return 0;

}

Compiler output:
a.cc: In function 'int main()':
a.cc:294:19: error: no matching function for call to 'sgn(Fraction<long long int>&, Fraction<long long int>&)'
  294 |             if(sgn(x,y) < 0){
      |                ~~~^~~~~
a.cc:162:5: note: candidate: 'int sgn(Z, Z)'
  162 | int sgn(const Z a, const Z b){
      |     ^~~
a.cc:162:17: note:   no known conversion for argument 1 from 'Fraction<long long int>' to 'Z' {aka 'long int'}
  162 | int sgn(const Z a, const Z b){
      |         ~~~~~~~~^
a.cc:172:5: note: candidate: 'int sgn(Q, Q)'
  172 | int sgn(Q a, Q b){
      |     ^~~
a.cc:172:11: note:   no known conversion for argument 1 from 'Fraction<long long int>' to 'Fraction<long int>'
  172 | int sgn(Q a, Q b){
      |         ~~^