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

namespace ARC066C
{
    public class Program
    {
        void Solve(StreamScanner ss, StreamWriter sw)
        {
            //---------------------------------
            var N = ss.Next(int.Parse);
            var A = ss.Next(int.Parse, N);
            Array.Sort(A);

            for (var i = 0; i < N; i++)
            {
                if (A[i] == (N % 2 == 0 ? i + 1 - i % 2 : i + i % 2)) continue;
                sw.WriteLine(0);
                return;
            }

            sw.WriteLine(new ModInt(2).Pow(N / 2));
            //---------------------------------
        }

        static void Main()
        {
            var ss = new StreamScanner(new StreamReader(Console.OpenStandardInput()));
            var sw = new StreamWriter(Console.OpenStandardOutput()) {AutoFlush = false};
            new Program().Solve(ss, sw);
            sw.Flush();
        }

        static readonly Func<string, string> String = s => s;
    }

    public struct ModInt
    {
        public static int Mod = 1000000007;

        readonly long value;
        public int Value { get { return (int)value; } }
        public ModInt Inverse { get { return GetInverse(); } }

        public ModInt(long value)
        {
            value %= Mod;
            this.value = value < 0 ? value + Mod : value;
        }

        ModInt GetInverse()
        {
            if (value == 0) throw new InvalidOperationException("value must NOT equal 0");
            return Pow(Mod - 2);
        }

        public ModInt Square()
        {
            return this * this;
        }

        public ModInt Pow(long exp)
        {
            if (exp == 0) return 1;
            if (exp % 2 == 0) return Pow(exp / 2).Square();
            return this * Pow(exp - 1);
        }

        public static ModInt Parse(string str)
        {
            return long.Parse(str);
        }

        public static implicit operator ModInt(long value)
        {
            return new ModInt(value);
        }

        public static ModInt operator +(ModInt left, ModInt right)
        {
            return left.value + right.value;
        }

        public static ModInt operator -(ModInt left, ModInt right)
        {
            return left.value - right.value;
        }

        public static ModInt operator *(ModInt left, ModInt right)
        {
            return left.value * right.value;
        }

        public static ModInt operator /(ModInt left, ModInt right)
        {
            return left * right.Inverse;
        }

        public override bool Equals(object obj)
        {
            if (!(obj is ModInt)) return false;
            return value == ((ModInt)obj).value;
        }

        public override int GetHashCode()
        {
            return value.GetHashCode();
        }

        public override string ToString()
        {
            return value.ToString();
        }
    }

    public class StreamScanner
    {
        static readonly char[] Sep = {' '};
        readonly Queue<string> buffer = new Queue<string>();
        readonly TextReader textReader;

        public StreamScanner(TextReader textReader)
        {
            this.textReader = textReader;
        }

        public T Next<T>(Func<string, T> parser)
        {
            if (buffer.Count != 0) return parser(buffer.Dequeue());
            var nextStrings = textReader.ReadLine().Split(Sep, StringSplitOptions.RemoveEmptyEntries);
            foreach (var nextString in nextStrings) buffer.Enqueue(nextString);
            return Next(parser);
        }

        public T[] Next<T>(Func<string, T> parser, int x)
        {
            var ret = new T[x];
            for (var i = 0; i < x; ++i) ret[i] = Next(parser);
            return ret;
        }

        public T[][] Next<T>(Func<string, T> parser, int x, int y)
        {
            var ret = new T[y][];
            for (var i = 0; i < y; ++i) ret[i] = Next(parser, x);
            return ret;
        }
    }
}