Problem:
Score : 500 points

Problem Statement

Given are N integers A_1,\ldots,A_N.

We will choose exactly K of these elements. Find the maximum possible product of the chosen elements.

Then, print the maximum product modulo (10^9+7), using an integer between 0 and 10^9+6 (inclusive).

Constraints

1 \leq K \leq N \leq 2\times 10^5

|A_i| \leq 10^9

Input

Input is given from Standard Input in the following format:

N K
A_1 \ldots A_N

Output

Print the maximum product modulo (10^9+7), using an integer between 0 and 10^9+6 (inclusive).

Sample Input 1

4 2
1 2 -3 -4

Sample Output 1

12

The possible products of the two chosen elements are 2, -3, -4, -6, -8, and 12, so the maximum product is 12.

Sample Input 2

4 3
-1 -2 -3 -4

Sample Output 2

1000000001

The possible products of the three chosen elements are -24, -12, -8, and -6, so the maximum product is -6.

We print this value modulo (10^9+7), that is, 1000000001.

Sample Input 3

2 1
-1 1000000000

Sample Output 3

1000000000

The possible products of the one chosen element are -1 and 1000000000, so the maximum product is 1000000000.

Sample Input 4

10 10
1000000000 100000000 10000000 1000000 100000 10000 1000 100 10 1

Sample Output 4

999983200

Be sure to print the product modulo (10^9+7).

Solution in C# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        Console.SetOut(sw);
        Solve();
        Console.Out.Flush();
    }
    static void Solve()
    {
        var N = sc.ReadInt();
        var K = sc.ReadInt();
        var A = sc.ReadLongArray(N);
        Array.Sort(A, (a, b) => Math.Abs(a).CompareTo(Math.Abs(b)));
        Array.Reverse(A);
        var P = new List<long>();
        var M = new List<long>();
        for (int i = 0; i < N; i++)
        {
            if (A[i] == 0) continue;
            if (A[i] < 0) M.Add(-A[i]);
            else P.Add(A[i]);
        }

        // ゼロが答え
        if (A.Count(a => a != 0) < K)
        {
            Console.WriteLine(0);
            return;
        }

        var mod = (int)1e9 + 7;

        // 正の数が答え
        var val1 = 1L;
        var modCount1 = 0L;
        var isMinus = false;
        var lastIndex = -1;
        var index = 0;
        foreach (var a in A.Where(a => a != 0).Take(K))
        {
            if (a > 0) lastIndex = index;
            index++;
            if (a < 0) isMinus = !isMinus;
            val1 *= a > 0 ? a : -a;
            if (val1 >= mod)
            {
                modCount1 += val1 / mod;
                val1 %= mod;
            }
        }
        if (!isMinus)
        {
            Console.WriteLine(val1);
            return;
        }

        // 負の数になったとき
        var ok = false;
        var val2 = 1L;
        var modCount2 = 0L;
        index = 0;
        foreach (var a in A.Where(a => a != 0).Take(K))
        {
            index++;
            if (index == lastIndex + 1) continue;

            val2 *= a > 0 ? a : -a;
            if (val2 >= mod)
            {
                modCount2 += val2 / mod;
                val2 %= mod;
            }
        }
        foreach (var a in A.Where(a => a != 0).Skip(K))
        {
            val2 *= a > 0 ? a : -a;
            if (val2 >= mod)
            {
                modCount2 += val2 / mod;
                val2 %= mod;
            }
            ok = true;
        }
        if (lastIndex != -1 && ok)
        {
            if ((modCount1, val1).CompareTo((modCount2, val2)) >= 0)
            {
                Console.WriteLine(val1);
            }
            else
            {
                Console.WriteLine(val2);
            }
            return;
        }

        // 負の数になりそうなとき
        if (A.Contains(0))
        {
            Console.WriteLine(0);
            return;
        }
        ModInt ans = 1;
        Array.Reverse(A);
        for (int i = 0; i < K; i++)
        {
            ans *= A[i];
        }
        Console.WriteLine(ans);
    }
    public struct ModInt
    {
        public const long Mod = (int)1e9 + 7;
        public long num;
        public ModInt(long n) { num = n; while (num < 0) num += Mod; }
        public override string ToString() { return num.ToString(); }
        public static ModInt operator +(ModInt l, ModInt r) { l.num += r.num; if (l.num >= Mod) l.num -= Mod; return l; }
        public static ModInt operator -(ModInt l, ModInt r) { l.num -= r.num; if (l.num < 0) l.num += Mod; return l; }
        public static ModInt operator *(ModInt l, ModInt r) { return new ModInt(l.num * r.num % Mod); }
        public static ModInt operator /(ModInt l, ModInt r) { return l * Inverse(r); }
        public static implicit operator ModInt(long n) { n %= Mod; if (n < 0) n += Mod; return new ModInt(n); }
        public static ModInt Pow(ModInt v, long k) { return Pow(v.num, k); }
        public static ModInt Pow(long v, long k)
        {
            v = new ModInt(v).num;
            long res = 1;
            while (k > 0)
            {
                if ((k & 1) == 1) res = res * v % Mod;
                v = v * v % Mod;
                k >>= 1;
            }
            return new ModInt(res);
        }
        public static ModInt Inverse(ModInt v) { return Pow(v, Mod - 2); }
        private static List<ModInt> Facs = new List<ModInt> { 1 };
        private static List<ModInt> iFacs = new List<ModInt> { 1 };
        public static ModInt Fac(int n)
        {
            for (int i = Facs.Count; i <= n; i++)
            {
                Facs.Add(i * Facs[i - 1]);
                iFacs.Add(Inverse(Facs[i]));
            }
            return Facs[n];
        }
        public static ModInt iFac(int n)
        {
            Fac(n);
            return iFacs[n];
        }
        public static ModInt nCr(int n, int r, bool loop = false)
        {
            if (n < r || n < 0 || r < 0) return 0;
            if (n == r) return 1;
            if (loop && n > (int)1e6) return nCrLoop(n, r);
            return Fac(n) * iFac(r) * iFac(n - r);
        }
        public static ModInt nCrLoop(long n, long r)
        {
            ModInt num = 1;
            for (long i = 1; i <= r; i++)
            {
                num = num * (n - i + 1) / i;
            }
            return num;
        }
        public static ModInt nHr(int n, int r)
        {
            return nCr(n + r - 1, r);
        }
    }
    static Scanner sc = new Scanner();
}
class Scanner
{
    string[] S = new string[0];
    int Index = 0;
    char[] Separators = new char[] { ' ' };

    public string Next()
    {
        if (this.Index < this.S.Length) return this.S[this.Index++];
        var line = "";
        while (line == "") line = Console.ReadLine();
        this.S = line.Split(this.Separators, StringSplitOptions.RemoveEmptyEntries);
        if (this.S.Length == 0) return this.Next();
        this.Index = 0;
        return this.S[this.Index++];
    }
    public string ReadStr()
    {
        return this.Next();
    }
    public char ReadChar()
    {
        return this.Next()[0];
    }
    public int ReadInt()
    {
        return int.Parse(this.Next());
    }
    public uint ReadUInt()
    {
        return uint.Parse(this.Next());
    }
    public long ReadLong()
    {
        return long.Parse(this.Next());
    }
    public double ReadDouble()
    {
        return double.Parse(this.Next());
    }
    public Tuple<int, int> ReadTup(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add);
    }
    public Tuple<long, long> ReadTupLong(int add = 0)
    {
        return Tuple.Create(this.ReadLong() + add, this.ReadLong() + add);
    }
    public Tuple<int, int, int> ReadTup3(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add);
    }
    public Tuple<int, int, int, int> ReadTup4(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add);
    }
    public int[] ReadIntArray(int n)
    {
        var array = new int[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadInt();
        }
        return array;
    }
    public long[] ReadLongArray(int n)
    {
        var array = new long[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadLong();
        }
        return array;
    }
    public double[] ReadDoubleArray(int n)
    {
        var array = new double[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadDouble();
        }
        return array;
    }
    public char[] ReadCharArray(int n)
    {
        var array = new char[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadChar();
        }
        return array;
    }
    public string[] ReadStrArray(int n)
    {
        var array = new string[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadStr();
        }
        return array;
    }
    public Tuple<long, long>[] ReadTupLongArray(int n, int add = 0)
    {
        var array = new Tuple<long, long>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTupLong(add);
        }
        return array;
    }
    public Tuple<int, int>[] ReadTupArray(int n, int add = 0)
    {
        var array = new Tuple<int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup(add);
        }
        return array;
    }
    public Tuple<int, int, int>[] ReadTup3Array(int n, int add = 0)
    {
        var array = new Tuple<int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup3(add);
        }
        return array;
    }
    public Tuple<int, int, int, int>[] ReadTup4Array(int n, int add = 0)
    {
        var array = new Tuple<int, int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup4(add);
        }
        return array;
    }
}