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 Haskell (GHC 7.10.3) (judge verdict: Accepted)

import Control.Monad
import qualified Data.ByteString.Char8 as BS
import Data.Maybe (fromJust)
import Data.List

modNum :: Integer
modNum = 10^9 + 7

canSolve :: [Integer] -> Bool
canSolve arr
    | isOdd = canSolveOdd sorted 0
    | otherwise = canSolveEven sorted 0
    where
        isOdd = ((length arr) `mod` 2) > 0
        sorted = sort arr

canSolveOdd :: [Integer] -> Integer -> Bool
canSolveOdd [] _ = True
canSolveOdd (x:xs) idx
    | x == target = canSolveOdd xs (idx + 1)
    | otherwise = False
    where
        target = ((idx + 1) `div` 2) * 2


canSolveEven :: [Integer] -> Integer -> Bool
canSolveEven [] _ = True
canSolveEven (x:xs) idx
    | x == target = canSolveEven xs (idx + 1)
    | otherwise = False
    where
        target = (idx `div` 2) * 2 + 1

modPow :: Integer -> Integer -> Integer
modPow _ 0 = 1
modPow x y = (x * (modPow x (y - 1))) `mod` modNum

solve:: [Integer] -> Integer
solve arr
    | (canSolve arr) = modPow 2 (fromIntegral ((length arr) `div` 2))
    | otherwise = 0

main :: IO ()
main = do
    _ <- BS.getLine
    a <- map (fst . fromJust . BS.readInteger) . BS.words <$> BS.getLine
    print $ solve a