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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;
import java.awt.geom.Point2D;
import java.awt.*;
import static java.lang.System.*;
import static java.lang.Math.*;
//Loopで使う文字列の長さは固定！
//intで大丈夫？オーバーフローしない？
//Loop回数は本当に10⁹以内になってる？Loopは回る？？
public class Main {
    public static void main(String[]$){
        Scanner sc = new Scanner(in);
        int mod=1000000007;
        int n=sc.nextInt();
        int[] a=new int[n];
        for (int i = 0; i < n; i++) {
            a[i]=sc.nextInt();
        }
        HashMap<Integer,Integer> map=new HashMap<>();
        for (int i = 0; i < n; i++) {
            map.put(a[i],map.getOrDefault(a[i], 0)+1);
        }
        if((n%2==0&&map.get(0)!=null)||(n%2==1&&map.get(0)!=1)){
            out.println(0);
            exit(0);
        }
        for(int k:map.keySet()){
            if(k!=0&&map.get(k)!=2){
                out.println(0);
                exit(0);
            }
        }
        if(n%2==0){
            out.println(power(2,n/2)%mod);
        }else{
            out.println(power(2,(n-1)/2)%mod);
        }
    }
    static long power(long x,int n){
        long mod=1000000007;
        long ans=1;
        while (n>0) {
            if((n&1)==1){
                ans=(ans*x)%mod;
            }
            x=(x*x)%mod;
            n>>=1;
        }
        return ans;
    }
}