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.Linq;
using System.Collections.Generic;
using System.Text;
using static System.Console;
using static System.Math;

class Program
{
    static void Main(string[] args)
    {
        solve();
    }
    static Scanner cin;

    static int N;
    static int[] a;
    static void input()
    {
        cin = new Scanner();
        N = cin.nextInt();
        a = cin.ArrayInt(N);
    }
    static void solve()
    {
        input();
        Dictionary<int, int> dic = new Dictionary<int, int>();
        if (N % 2 == 1)
            dic.Add(0, 1);
        for(int i = 0; i < N; i++)
        {
            int A = a[i];
            if (dic.ContainsKey(A))
            {
                dic[A]++;
            }
            else
            {
                dic.Add(A, 1);
            }
            if(dic[A] == 3)
            {
                WriteLine(0);
                return;
            }
        }
        Mod mod = new Mod(1000000007);
        WriteLine(mod.Pow(2, N / 2));
    }

    
    class Scanner
    {
        string[] s;
        int i;

        char[] cs = new char[] { ' ' };

        public Scanner()
        {
            s = new string[0];
            i = 0;
        }
        
        public string next()
        {
            if (i < s.Length) return s[i++];
            string st = Console.ReadLine();
            while (st == "") st = Console.ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            if (s.Length == 0) return next();
            i = 0;
            return s[i++];
        }

        public int nextInt()
        {
            return int.Parse(next());
        }

        public int[] ArrayInt(int N, int add = 0)
        {
            int[] Array = new int[N];
            for(int i = 0; i < N; i++)
            {
                Array[i] = nextInt() + add;
            }
            return Array;
        }

        public long nextLong()
        {
            return long.Parse(next());
        }

        public long[] ArrayLong(int N, long add = 0)
        {
            long[] Array = new long[N];
            for(int i = 0; i < N; i++)
            {
                Array[i] = nextLong() + add;
            }
            return Array;
        }

        public double nextDouble()
        {
            return double.Parse(next());
        }

        public double[] ArrayDounble(int N, int add = 0)
        {
            double[] Array = new double[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextDouble() + add;
            }
            return Array;
        }
    }
      public class Mod
    {
        public long modulo;
        public Mod(long _module)
        {
            modulo = _module;
        }

        public long Add(long n, long m)
        {
            return (n % modulo + m % modulo) % modulo;
        }

        public long Sub(long val, long minos)
        {
            long ret = val % minos;
            if (ret < 0) ret += minos;

            return ret;
        }
        public long Mul(long a, long b)
        {
            return ((a % modulo) * (b % modulo)) % modulo;
        }
        public long Pow(long b, long i)
        {
            long ret = 1;
            while(i > 0)
            {
                if ((i & 1) > 0)
                    ret = ret * b % modulo;

                b = b * b % modulo;
                i >>= 1;
            }
            return ret;
        }
        public long Inv(long val)
        {
            return Pow(val, modulo - 2);
        }
        public long Div(long val, long by)
        {
            return (val * Inv(by)) % modulo;
        }
    }
}