Problem:
Score : 300 points

Problem Statement

There are N people, conveniently numbered 1 through N.
They were standing in a row yesterday, but now they are unsure of the order in which they were standing.
However, each person remembered the following fact: the absolute difference of the number of the people who were standing to the left of that person, and the number of the people who were standing to the right of that person.
According to their reports, the difference above for person i is A_i.

Based on these reports, find the number of the possible orders in which they were standing.
Since it can be extremely large, print the answer modulo 10^9+7.
Note that the reports may be incorrect and thus there may be no consistent order.
In such a case, print 0.

Constraints

1≦N≦10^5

0≦A_i≦N-1

Input

The input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the number of the possible orders in which they were standing, modulo 10^9+7.

Sample Input 1

5
2 4 4 0 2

Sample Output 1

4

There are four possible orders, as follows:

2,1,4,5,3

2,5,4,1,3

3,1,4,5,2

3,5,4,1,2

Sample Input 2

7
6 4 0 2 4 0 2

Sample Output 2

0

Any order would be inconsistent with the reports, thus the answer is 0.

Sample Input 3

8
7 5 1 1 7 3 5 3

Sample Output 3

16

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Console;
using static System.Convert;
using static System.Math;
//using Debug;
//using static System.Globalization.CultureInfo;
//using Edge = Pair<long, int>;
using System.Text;
class Program
{
    static void Main(string[] args)
    {
        Solve();
        //WriteLine(Solve());
    }
    static void Solve()
    {
        var num = Input.num;
        var ar = Input.ar.OrderBy(v=>v).ToArray();
        var dic = new Dictionary<int, int>();
        if (num % 2 == 0)
        {
            var t = 1;
            for (var i = 0; i < num; i++)
                if (ar[i] != t)
                { WriteLine(0); return; }
                else if (i % 2 == 1) t += 2;
            WriteLine(Modulo.Pow(2, num/2));
        }
        else
        {
            if(ar[0]!=0)
            { WriteLine(0);return; }
            var t = 2;
            for (var i = 1; i < num; i++)
                if (ar[i] != t)
                {
                    WriteLine(0); return;
                }
                else if (i % 2 == 0) t += 2;
            WriteLine(Modulo.Pow(2, num / 2));
        }
    }
}

public class Input
{
    public static string read => ReadLine().Trim();
    public static int[] ar => read.Split(' ').Select(int.Parse).ToArray();
    public static int num => ToInt32(read);
    public static long[] arL => read.Split(' ').Select(long.Parse).ToArray();
    public static long numL => ToInt64(read);
    public static char[][] gred(int h)
        => Enumerable.Repeat(0, h).Select(_ => read.ToCharArray()).ToArray();
    public static int[] ar1D(int n)
        => Enumerable.Repeat(0, n).Select(_ => num).ToArray();
    public static long[] arL1D(int n)
        => Enumerable.Repeat(0, n).Select(_ => numL).ToArray();
    public static string[] strs(int n)
        => Enumerable.Repeat(0, n).Select(_ => read).ToArray();
    public static int[][] ar2D(int n)
        => Enumerable.Repeat(0, n).Select(_ => ar).ToArray();
    public static long[][] arL2D(int n)
        => Enumerable.Repeat(0, n).Select(_ => arL).ToArray();
    public static T getValue<T>(string g)
    {
        var t = typeof(T);
        if (t == typeof(int))
            return (T)(object)int.Parse(g);
        if (t == typeof(long))
            return (T)(object)long.Parse(g);
        if (t == typeof(string))
            return (T)(object)g;
        if (t == typeof(char))
            return (T)(object)char.Parse(g);
        if (t == typeof(double))
            return (T)(object)double.Parse(g);
        if (t == typeof(bool))
            return (T)(object)bool.Parse(g);
        return default(T);
    }
    public static string[] Make<T1, T2>(out T1 v1, out T2 v2)
    {
        var r = read.Split(' ');
        v1 = getValue<T1>(r[0]);
        v2 = getValue<T2>(r[1]);
        return r;
    }
    public static string[] Make<T1, T2, T3>(out T1 v1, out T2 v2, out T3 v3)
    {
        var r = Make(out v1, out v2);
        v3 = getValue<T3>(r[2]);
        return r;
    }
    public const long Inf = (long)1e18;
    public const double eps = 1e-6;
    public const string Alfa = "abcdefghijklmnopqrstuvwxyz";
    public const int MOD = 1000000007;
}
public class Modulo
{
    private static int Multiple(int num1, int num2)
        => (int)(BigMul(num1, num2) % Input.MOD);

    public static int Pow(int m, int n)
    {
        if (n == 0) return 1;
        if (n % 2 == 0) return Pow(Multiple(m, m), n >> 1);
        else return Multiple(Pow(Multiple(m, m), n >> 1), m);
    }

    public static int Div(int a, int b)
        => Multiple(a, Pow(b, Input.MOD - 2));

    public class Combination
    {
        private int[] _fac;
        public Combination(int num)//階乗をメモ化
        {
            _fac = new int[num + 1];
            _fac[0] = 1;
            for (var i = 1; i <= num; i++)
                _fac[i] = Multiple(_fac[i - 1], i);
        }
        public int Comb(int n, int r)
        {
            if (n < r) return 0;
            if (n == r) return 1;
            var calc = _fac[n];
            calc = Div(calc, _fac[r]);
            calc = Div(calc, _fac[n - r]);
            return calc;
        }
        public int fac(int num)
            => _fac[num];
    }

}