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 (Python):
n,*aa = map(int, open(0).read().split())

from collections import Counter
c = Counter(aa)

MOD = 10**9+7
cand = set(range((n+1)%2, n ,2))

if c.keys() - cand:
    ans = 0
else:
    ans = 1
    for v in cand:
        if v != 0 and c[v] != 2:
            ans = 0
            break
        elif v == 0 and c[v] != 1:
            ans = 0
            break
        elif v != 0:
            ans *= 2
            ans %= MOD

print(ans)

Solution (Python):
from collections import Counter

N = int(input())
A = list(map(int, input().split()))
cnt = Counter(A)
MOD = 10 ** 9 + 7
isAble = True
if N % 2 == 1:
    if cnt[0] != 1:
        isAble = False
    for i in range(2, N, 2):
        if cnt[i] != 2:
            isAble = False
            break
else:
    for i in range(1, N, 2):
        if cnt[i] != 2:
            isAble = False
            break

if isAble:
    ans = 1
    for _ in range(N // 2):
        ans *= 2
        ans %= MOD
    print(ans)
else:
    print(0)

Solution (Python):
n=int(input())
a=sorted(list(map(int,input().split())))[::-1]
mod=10**9+7
ans=1
j=a[0]
for i in range(1,n):
  if i%2:
    if a[i]!=j:
      print(0)
      break
    else:ans=2*ans%mod
  else:j=a[i]
else:print(ans%mod)

Solution (Python):
import sys

sys.setrecursionlimit(10 ** 7)
input = sys.stdin.readline
f_inf = float('inf')
mod = 10 ** 9 + 7


def resolve():
    n = int(input())
    A = sorted(list(map(int, input().split())))
    if n % 2 == 0:
        num = 1
        for i in range(0, n - 1, 2):
            if A[i] != num or A[i + 1] != num:
                print(0)
                exit()
            num += 2
    else:
        if A[0] != 0:
            print(0)
            exit()
        num = 2
        for i in range(1, n - 1, 2):
            if A[i] != num or A[i + 1] != num:
                print(0)
                exit()
            num += 2

    res = pow(2, n // 2, mod)
    print(res % mod)


if __name__ == '__main__':
    resolve()

Solution (Python):
from math import ceil,floor,comb,factorial,gcd,sqrt,log2,cos,sin,tan,acos,asin,atan,degrees,radians,pi,inf
from itertools import accumulate,permutations,combinations,product,combinations_with_replacement
from collections import deque,defaultdict,Counter
from bisect import bisect_left,bisect_right
from operator import itemgetter
from heapq import heapify,heappop,heappush
from queue import Queue,LifoQueue,PriorityQueue
from copy import deepcopy
from time import time
import string
import sys
sys.setrecursionlimit(10 ** 7)
def input() : return sys.stdin.readline().strip()
def INT()   : return int(input())
def MAP()   : return map(int,input().split())
def LIST()  : return list(MAP())

n = INT()
a = sorted(LIST())

chk = True
if n % 2 == 0:
    for i in range(n):
        if a[i] != i//2 * 2 + 1:
            chk = False
else:
    for i in range(n):
        if a[i] != (i+1)//2 * 2:
            chk = False
if chk:
    ans = pow(2, n//2, 10**9+7)
    print(ans)
else:
    print(0)

Solution (Python):
N = int(input())
A = list(map(int, input().split()))
M = 10**9+7

from collections import Counter
cd = Counter(A)


if N & 1:
    
    if cd[0] != 1:
        print(0)
        exit(0)
    ng = False

    for ix in range(2, N//2 + 1, 2):
        if cd[ix] != 2:
            ng = True
            break

    if len(cd) != N//2 + 1:
        ng = True
    
    if ng:
        print(0)
    else:
        
        ans = 1
        for ix in range(N//2):
            ans *= 2
            ans %= M
        print(ans)

else:
    
    ng = False
    for ix in range(1, N//2+1, 2):
        if cd[ix] != 2:
            ng = True
            break
    
    if len(cd) != N//2:
        ng = True
    
    if ng:
        print(0)
    else:
        
        ans = 1
        for ix in range(N//2):
            ans *= 2
            ans %= M
        print(ans)

Solution (Python):
N = int(input())
A = list(map(int, input().split()))
M = 10**9+7

from collections import Counter
cd = Counter(A)


if N & 1:
    
    if cd[0] != 1:
        print(0)
        exit(0)
    ng = False

    for ix in range(2, N//2 + 1, 2):
        if cd[ix] != 2:
            ng = True
            break

    if len(cd) != N//2 + 1:
        ng = True
    
    if ng:
        print(0)
    else:
        print(pow(2,(N//2),M))

else:
    
    ng = False
    for ix in range(1, N//2+1, 2):
        if cd[ix] != 2:
            ng = True
            break
    
    if len(cd) != N//2:
        ng = True
    
    if ng:
        print(0)
    else:
        print(pow(2,(N//2),M))

Solution (Python):



n = int(input())
a = list(map(int, input().split()))
d = {}
MOD = 10**9 + 7
for av in a:
    if av not in d:
        d[av] = 1
    else:
        d[av] += 1
if n % 2 == 0:
    ans = 1
    for i in range(1, n, 2):
        if i not in d or d[i] != 2:
            print(0)
            exit()
        else:
            ans *= 2
            ans %= MOD
    print(ans)
else:
    if 0 not in d or d[0] != 1:
        print(0)
        exit()
    ans = 1
    for i in range(2, n, 2):
        if i not in d or d[i] != 2:
            print(0)
            exit()
        else:
            ans *= 2
            ans %= MOD
    print(ans)

Solution (Python):
from collections import Counter

N = int(input())
A = list(map(int, input().split()))

cnt = Counter(A)
diff = {}
if N % 2 == 0:
    for i in range(1, N, 2):
        diff[i] = 2
else:
    for i in range(0, N, 2):
        if i == 0:
            diff[i] = 1
        else:
            diff[i] = 2
MOD = 10 ** 9 + 7
if cnt != diff:
    print(0)
    exit()
else:
    print(pow(2, N//2, MOD))

Solution (Python):
def count(l):
    res = {}
    for d in l:
        if d in res:
            res[d] += 1
        else:
            res[d] = 1
    return res

n = int(input())
data = list(map(int, input().split()))
cnt = count(data)
mod = 10**9 + 7

ans = 1
for k, v in cnt.items():
    if k == 0:
        if v > 1 or n %2 == 0:
            print(0)
            exit()
        continue
    else:
        if v <= 2:
            ans *= 2
        else:
            print(0)
            exit()

print(ans%mod)

Solution (Python):
from collections import *
from itertools import *
from math import *
n=int(input())
a=sorted(list(map(int,input().split())))
b=list(Counter(a))
mod=7+10**9
if n%2==1:
    if a[0]!=0:
        print(0)
        exit()
    for i in range(1,n):
        if ceil(i/2)*2!=a[i]:
            print(0)
            exit()
    print(2**(n//2)%mod)
if n%2==0:
    for i in range(n):
        if floor(i/2)*2+1!=a[i]:
            print(0)
            exit()
    print(2**(n//2)%mod)

Solution (Python):
import sys
from collections import Counter

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline

in_n = lambda: int(readline())
in_nn = lambda: map(int, readline().split())
in_nl = lambda: list(map(int, readline().split()))
in_na = lambda: map(int, read().split())
in_s = lambda: readline().rstrip().decode('utf-8')


def main():
    N = in_n()
    A = in_nl()
    count = Counter(A)

    mod = 10**9 + 7

    if N % 2 == 0:
        f = True
        for i in range(1, N, 2):
            if count[i] != 2:
                f = False
                break

        if f:
            ans = pow(2, N // 2, mod)
        else:
            ans = 0
    else:
        f = (count[0] == 1)
        for i in range(2, N, 2):
            if count[i] != 2:
                f = False
                break

        if f:
            ans = pow(2, N // 2, mod)
        else:
            ans = 0

    print(ans)


if __name__ == '__main__':
    main()

Solution (Python):
from collections import Counter
n = int(input())
A = list(map(int, input().split()))
cA = Counter(A)
mod = pow(10,9)+7
ans = pow(2,n//2)
for i in range(n//2+n%2):
    if i==0 and n%2==1:
        if cA[0] !=1:
            ans = 0
            break
    elif cA[2*i+1-n%2]!=2:
        ans = 0
        break
print(ans%mod)

Solution (Python):
N = int(input())
A = list(map(int, input().split()))

P = [0]*N

for i in range (0, N):
	P[A[i]]+=1

if P[0] != 1 and P[0]!=0:
	print(0)
	exit()

for i in range (1, N):
	if (i-N)%2 ==0:
		if P[i] > 0:
			print(0)
			exit()
	if (i-N)%2 != 0:
		if P[i] != 2 and P[i] !=0:
			print(0)
			exit()

print((2**(N//2))%(10**9+7))

Solution (Python):
n=int(input())
a=list(map(int,input().split()))
mod=10**9+7

a=sorted(a)
flgs=[]
if n%2==0:
    k=n//2
    for i in range(1,k+1):
        flgs.append(2*i-1)
        flgs.append(2*i-1)
else:
    k=(n-1)//2
    flgs.append(0)
    for j in range(1,k+1):
        flgs.append(2*j)
        flgs.append(2*j)

flgs=sorted(flgs)
if flgs==a:
    ans=2**k
    print(ans%mod)
else:
    print(0)

Solution (Python):
from collections import Counter

N=int(input())
A=list(map(int, input().split()))
D=Counter(A)
ans=1
p=10**9+7
for k,v in D.items():

    if k==0:
        if v!=1:
            print(0)
            exit()
    else:
        if v!=2:
            print(0)
            exit()
        else:
            ans*=2
            ans%=p
print(ans)

Solution (Python):
from collections import Counter

N = int(input())
As = list(map(int, input().split()))

P = 10**9 + 7

rlt = 1
dic = Counter(As)

if N % 2 == 0:
  for k in dic:
    if k % 2 == 1 and dic[k] == 2:
      rlt *= 2
      rlt %= P
    else:
      print(0)
      exit()
else:
  for k in dic:
    if k == 0 and dic[k] == 1:
      continue
    elif k % 2 == 0 and dic[k] == 2:
      rlt *= 2
      rlt %= P
    else:
      print(0)
      exit()
      
print(rlt)

Solution (Python):
n = int(input())
A = list(map(int,input().split()))

A.sort()

mod = 10**9+7

f = True
if n%2:
    if A[0] != 0:
        f = False
    else:
        check = 2
        for i in range(1,(n-1)//2+1):
            if not (A[2*i-1] == check and A[2*i] == check):
                f = False
                break
            check += 2
else:
    check = 1
    for i in range(1,n//2+1):
        if not (A[2*i-2] == check and A[2*i-1] == check):
            f = False
            break
        check += 2
if f:
    if n%2:
        print(pow(2, (n-1)//2, mod))
    else:
        print(pow(2, n//2, mod))
else:
    print(0)

Solution (Python):


import sys
read = sys.stdin.read
readline = sys.stdin.readline

n,*a = map(int,read().split())

MOD = 10**9+7


res = [0]*(n)
for i in a:
    res[i] += 1

if n&1:
    if res[0]==1 and all(ri==2 for ri in res[2::2]):
        print(pow(2,n//2,MOD))
    else:
        print(0)
else:
    if all(ri==2 for ri in res[1::2]):
        print(pow(2,n//2,MOD))
    else:
        print(0)

Solution (Python):
import sys
from collections import Counter

N = int(input())
A = list(map(int, sys.stdin.readline().rsplit()))
mod = 10 ** 9 + 7

C = Counter(A)
if N % 2 == 0:
    if not all([i == 2 for i in C.values()]):
        print(0)
    else:
        print(pow(2, N // 2, mod))
else:
    for i, v in C.items():
        if v > 2:
            print(0)
            exit()
        elif i != 0 and v == 1:
            print(0)
            exit()
        elif i == 0 and v > 1:
            print(0)
            exit()
    if 0 in C:
        print(pow(2, N // 2, mod))
    else:
        print(0)

Solution (Python):
N = int(input())
A = list(map(int,input().split()))
if N==1:
    if A[0]==0:
        print(1)
    else:
        print(0)
else:
    C = {}
    for i in range(N):
        a = A[i]
        if a not in C:
            C[a]=0
        C[a] += 1
    flag = 0
    if N%2==0:
        for i in range(1,N,2):
            if i in C and C[i]==2:continue
            flag=1
            break
    else:
        for i in range(2,N,2):
            if i in C and C[i]==2:continue
            flag = 1
            break
        if 0 not in C or C[0]!=1:
            flag = 1
    if flag==1:
        print(0)
    else:
        p = 10**9+7
        k = N//2
        ans = 1
        cnt = 1
        while cnt<=k:
            ans = (ans*2)%p
            cnt += 1
        print(ans)

Solution (Python):
from collections import Counter
def solve():
    ans = 0
    mod = 10**9+7
    N = int(input())
    A = list(map(int, input().split()))
    c = Counter(A)
    for i in range(N):
        if (N-1-i)==i:
            if 0 not in c or c[0]!=1:
                return 0
        else:
            p = abs((N-1-i)-i)
            if p not in c or c[p]!=2:
                return 0
    ans = pow(2,N//2,mod)
    return ans
print(solve())

Solution (Python):
n = int(input())
a = list(map(int, input().split()))
a.sort()
mod = 10**9+7

def pow(x, n):
    ans = 1
    n_tmp = n
    while n:
        if n % 2:
            ans *= x
            ans %= mod
        x *= x
        n >>= 1
    return ans

if n % 2 == 0:
    for i in range(n):
        if a[i] != 2*(i//2)+1:
            print(0)
            exit()
    print(pow(2, n//2))
else:
    for i in range(n):
        if a[i] != 2*((i+1)//2):
            print(0)
            exit()
    print(pow(2, n//2))

Solution (Python):
from collections import Counter
mod = 10 ** 9 + 7


def main():
    N = int(input())
    A = list(map(int, input().split()))
    C = Counter(A)

    
    if N % 2 == 1:
        if C[0] != 1:
            return 0
        for i in range(2, N, 2):
            if C[i] != 2:
                return 0
    else:
        for i in range(1, N, 2):
            if C[i] != 2:
                return 0
    return pow(2, N//2, mod)


if __name__ == "__main__":
    print(main())

Solution (Python):
N = int(input())
A = list(map(int,input().split()))
mod = 10**9+7

cnd = []
if N%2 == 0:
    for i in range(N//2):
        cnd.append(2*i+1)
        cnd.append(2*i+1)
else:
    for i in range(N//2+1):
        if i == 0:
            cnd.append(0)
        else:
            cnd.append(2*i)
            cnd.append(2*i)
            
B = sorted(A)
if cnd != B:
    print(0)
    exit()

print(pow(2,N//2,mod))

Solution (Python):
n = int(input())
A = list(map(int, input().split()))

mod = 10**9+7

def power(a, n, mod):
    bi=str(format(n,"b")) 
    res=1
    for i in range(len(bi)):
        res=(res*res) %mod
        if bi[i]=="1":
            res=(res*a) %mod
    return res


from collections import Counter
if n%2 == 0:
    C = Counter(A)
    for i in range(1, n+1):
        if i%2 == 0:
            continue
        else:
            if C[i] != 2:
                print(0)
                exit()
    else:
        print(power(2, n//2, mod))
else:
    C = Counter(A)
    for i in range(1, n+1):
        if i == 0:
            if C[i] != 1:
                print(0)
                exit()
        elif i%2 == 1:
            continue
        else:
            if C[i] != 2:
                print(0)
                exit()
    else:
        print(power(2, n//2, mod))

Solution (Python):
import sys
input = sys.stdin.readline
from collections import Counter

def read():
    N = int(input().strip())
    A = list(map(int, input().strip().split()))
    return N, A


def solve(N, A, p=10**9+7):
    A.sort()
    if N % 2 == 1:
        if A[0] != 0:
            return 0
        for i in range(1, N):
            if A[i] != i + i%2:
                return 0
    else:
        for i in range(0, N):
            if A[i] != i + (1-i%2):
                return 0
    return pow(2, N//2, p)


if __name__ == '__main__':
    inputs = read()
    outputs = solve(*inputs)
    if outputs is not None:
        print("%s" % str(outputs))

Solution (Python):
import collections
N=int(input())
A=collections.Counter(list(map(int,input().split())))
if N%2==0:
  for i in range(0,N,2):
    if i%2==0 and A[i]==0:
      pass
    elif i%2==1 and A[i]==2:
      pass
    else:
        print(0);exit(0)
else:
  for i in range(N):
    if i==0 and A[i]==1:
      continue
    if i%2==0 and A[i]==2:
      pass
    elif i%2==1 and A[i]==0:
      pass
    else:
      print(0);exit(0)
print(pow(2,N//2,10**9+7 ))

Solution (Python):
from collections import Counter

def powmod(a,n,m): 
    ans=1
    while n!=0:
        if n&1: ans=ans*a%m
        a=a*a%m
        n>>=1
    return ans%m

n=int(input())
a=list(map(int,input().split()))
a.sort()
dic=Counter(a)
if n%2==0:
    temp={}
    for i in range(1,n,2):
        temp[i]=2
    if temp!=dict(dic):
        print(0)
        exit()
else:
    temp={}
    temp[0]=1
    for i in range(2,n,2):
        temp[i]=2
    if temp!=dict(dic):
        print(0)
        exit()
print(powmod(2,n//2,10**9+7))

Solution (Python):
import collections

n = int(input())
a = list(map(int, input().split()))

c = collections.Counter(a)

if n % 2 == 0:
    for i in range(n):
        if i % 2 == 0 and c[i] == 0:
            continue
        elif i % 2 == 1 and c[i] == 2:
            continue
        else:
            print(0)
            exit(0)
else:
    for i in range(n):
        if i == 0 and c[i] == 1:
            continue
        if i % 2 == 0 and c[i] == 2:
            continue
        elif i % 2 == 1 and c[i] == 0:
            continue
        else:
            print(0)
            exit(0)

print(pow(2, n // 2, 10 ** 9 + 7))

Solution (Python):
mod = 10**9+7
def power(a,n):
  ans = 1
  while n:
    if n%2:
      ans = (ans*a)%mod
    a = (a*a)%mod
    n >>= 1
  return ans%mod 
n = int(input())
a = list(map(int,input().split()))
x = [0 for _ in range(n)]
for ai in a:
  x[ai] += 1
if n%2 == 0:
  if sum(x[0::2]) != 0:
    print(0)
    exit()
  for i in range(1,n,2):
    if x[i] != 2:
      print(0)
      exit()
  print(power(2,n//2))
else:
  if sum(x[1::2]) != 0:
    print(0)
    exit()
  if x[0] != 1:
    print(0)
    exit()
  if n == 1:
    print(1)
    exit()
  for i in range(2,n,2):
    if x[i] != 2:
      print(0)
      exit()
  print(power(2,(n-1)//2))

Solution (Python):
from collections import Counter

n = int(input())
al = list(map(int, input().split()))

c_al = Counter(al)

if n % 2 == 0:
    for k,v in c_al.items():
        if v != 2:
            print(0)
            exit()
    print(pow(2, n // 2, 10 ** 9 + 7))

elif n % 2 != 0:
    for k, v in c_al.items():
        if k == 0:
            if v != 1:
                print(0)
                exit()
        elif v != 2:
            print(0)
            exit()
    print(pow(2, n // 2, 10 ** 9 + 7))

Solution (Python):
def solve(A,N,M):
    
    
    
    

    A.sort(reverse = True)

    if N % 2 != 0:
        if A[len(A)-1] == 0:
            A.pop()
        else:
            return 0

    ans = pow(2,len(A)//2,M)
    a = 1
    if N % 2 != 0:
        a += 1
    while len(A) > 0:
        if A[len(A)-1] == a and A[len(A)-2] == a:
            A.pop()
            A.pop()
            a += 2
        else:
            return 0
    return ans

def main():
    N = int(input())
    A = list(map(int,input().split()))
    M = 10**9 + 7

    print(solve(A,N,M))

if __name__ == "__main__":
    main()

Solution (Python):
def solve(n,A):
    MOD = 10**9+7

    freq = [0 for i in range(n)]
    for x in A:
        freq[x] += 1

    ans = 1
    if n&1:
        if freq[0] != 1:
            return 0

    for pos in range(n//2):
        r = abs(pos-(n-1-pos))
        if freq[r]==0:
            ans = (ans*1)%MOD
        elif freq[r]==2:
            ans = (ans*2)%MOD
        else:
            return 0
    return ans

n = int(input())
A = [int(x) for x in input().split()]
print(solve(n,A))

Solution (Python):
from collections import Counter


def is_valid():
    if N % 2:
        return A[0] == 1 and all(A[2 * i + 2] == 2 for i in range(N // 2))
    else:
        return all(A[2 * i + 1] == 2 for i in range(N // 2))


N = int(input())
A = Counter(map(int, input().split()))
print(pow(2, N // 2, 10 ** 9 + 7)) if is_valid() else print(0)

Solution (Python):
from collections import Counter


def is_valid():
    if N % 2:
        return A.get(0, 0) == 1 and all(A.get(2 * i + 2, 0) == 2 for i in range(N // 2))
    else:
        return all(A.get(2 * i + 1, 0) == 2 for i in range(N // 2))


N = int(input())
A = Counter(map(int, input().split()))
MOD = 10 ** 9 + 7
print(pow(2, N // 2, MOD)) if is_valid() else print(0)

Solution (Python):
n=int(input())
a=list(map(int,input().split()))

if n==1:
  if a[0]==0:
    print(1)
    exit()
  exit(print(0))
mod=10**9+7
a.sort()
t=0
ans=1
if n%2==0:
  t=1
  for i in range(n):
    if i==0 or i==1:
      if a[i]!=1:exit(print(0))
      continue
    if i%2==0:t+=2
    if a[i]!=t:exit(print(0))
  for i in range(n//2):
    ans*=2
    ans%=mod
else:
  for i in range(n):
    if i==0 and a[i]!=0:exit(print(0))
    if i%2==1:
      t+=2
    if a[i]!=t:exit(print(0))
  for i in range(n//2):
    ans*=2
    ans%=mod
print(ans)

Solution (Python):
n,*a=map(int,open(0).read().split())
from collections import Counter
x=Counter(a)
MOD=10**9+7

if n%2==1:
  ng=False
  if x[0]!=1:ng=True
  for i in range(1,n):
    if i%2==0 and x[i]==2:continue
    if i%2==1 and x[i]==0:continue
    ng=True
  if ng:
    print(0)
  else:
    print(pow(2,n//2,MOD))

if n%2==0:
  ng=False
  for i in range(n):
    if i%2==0 and x[i]==0:continue
    if i%2==1 and x[i]==2:continue
    ng=True  
  
  if ng:
    print(0)
  else:
    print(pow(2,n//2,MOD))

Solution (Python):
n = int(input())
a = list(map(int,input().split()))
count = [0]*n
for i in a:
    count[i] += 1
    
ans = 0
if n%2 == 0:
    if all(count[2*f]==0 for f in range(n//2)) and all(count[2*k-1]==2 for k in range(1,n//2)):  
        ans = 1
else:
    if count[0] == 1 and all(count[2*f]==2 for f in range(1,n//2)) and all(count[2*k-1]==0 for k in range(1,n//2)):
        ans = 1
        
if ans == 1:
    for i in range(n//2):
        ans = (ans*2) % (1e+9+7)

print(int(ans))

Solution (Python):
from collections import Counter

n = int(input())
a = list(map(int, input().split()))
MOD = 10 ** 9 + 7

mono = sorted(set(a))
dic = Counter(a)

if n % 2 == 1:
    last = 0
else:
    last = 1

res = 0
Cond = True
for x in mono:
    if x == 0 and a.count(0) == 1: last = 2
    elif x == last and dic[x] == 2:
        res += 1
        last += 2
    else: Cond = False

if Cond:
    print(2**res % MOD)
else:
    print(0)

Solution (Python):
from collections import Counter
from sys import stdin


def main():
    input = lambda: stdin.readline()[:-1]
    N = int(input())
    A = list(map(int, input().split()))

    cnt = Counter(A)
    n = N
    if N % 2:
        if cnt[0] != 1:
            print(0)
            return
        n = N - 1
    for i in range(1, n, 2):
        if cnt[N-i] != 2:
            print(0)
            return
    print(pow(2, n // 2, 1000000007))


main()

Solution (Python):
MOD = 10**9+7

def mi():
    return map(int, input().split())

def ii():
    return int(input())

def main():
    N = ii()
    A = list(mi())
    A.sort()
    if N%2:
        for i in range(N):
            if A[i] != i+(i%2):
                print(0)
                return
    else:
        for i in range(N):
            if A[i] != i+((i+1)%2):
                print(0)
                return
    print(pow(2, N//2, MOD))


if __name__ == '__main__':
    main()

Solution (Python):
mod = 10**9+7

N = int(input())
A = list(map(int,input().split()))

chk = []
if N % 2 == 1:
    chk.append(0)
    for i in range(N // 2):
        chk.append((i+1)*2)
        chk.append((i+1)*2)
else:
    for i in range(N // 2):
        chk.append(i*2+1)
        chk.append(i*2+1)


A.sort()

if A != chk:
    print(0)
else:
    print(pow(2, N // 2,mod))

Solution (Python):
n = int(input())
a = list(map(int,input().split()))

from collections import Counter
cnt = Counter(a)
ans = 0
for i in range(n - 1, -1, -2):
    if cnt[i] == 2:
        ans += 1
        continue
    if i == 0 and cnt[i] == 1:
        break
    else:
        print(0)
        exit()
M = 10 ** 9 + 7
print((2 ** ans)%M)

Solution (Python):
import sys
from collections import Counter

sys.setrecursionlimit(10 ** 7)
rl = sys.stdin.readline


def solve():
    N = int(rl())
    A = list(map(int, rl().split()))
    MOD = 10 ** 9 + 7
    
    counter = Counter(A)
    if N % 2 == 0:
        for n in range(1, N, 2):
            if counter[n] != 2:
                print(0)
                return
    else:
        if counter[0] != 1:
            print(0)
            return
        for n in range(2, N, 2):
            if counter[n] != 2:
                print(0)
                return
    print(pow(2, N // 2, MOD))


if __name__ == '__main__':
    solve()

Solution (Python):
n=int(input())
a=map(int,input().split())

import collections
c = collections.Counter(a)
m=0
for i in range(n//2):
    if n//2==n/2:
        if c[n-1-2*i]!=2:
            m=1
    else:
        if (i!=n//2-1 and c[n-1-2*i]!=2):
            m=1
        if (i==n//2 and c[n-1-2*i]!=1):
            m=1
if m==0:
    print(2**(n//2)%(10**9+7))
else:
    print(0)

Solution (Python):
n = int(input())
a = list(map(int, input().split()))
if n % 2 == 0:
    l = [2 * i + 1 for i in range(n // 2)] * 2
else:
    l = [2 * i for i in range(1, n // 2 + 1)] + [2 * i for i in range(n // 2 + 1)]
a.sort()
l.sort()
if a != l:
    print(0)
else:
    print(pow(2, n // 2, 10 ** 9 + 7))

Solution (Python):
import sys
import copy
from collections import deque
stdin = sys.stdin

mod = 10**9+7

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().rstrip()  

n = ni()
a_list = na()

c = {abs((n-1-i)-i): 0 for i in range(n//2 + 1)}

for a in a_list:
    if a not in c or c[a]==2:
        print(0)
        exit(0)
    c[a] += 1

cnt = 1
for k,v in c.items():
    if v==2 or k==0:
        cnt *= v
        cnt %= mod
    else:
        print(0)
        exit(0)

print(cnt)

Solution (Python):
n = int(input())
A = sorted(map(int, input().split()))
MOD = 10 ** 9 + 7
if n % 2:
    if A[0] != 0:
        print(0)
        exit()
    for i in range(1, n-1, 2):
        if A[i] != i + 1 or A[i] != A[i+1]:
            print(0)
            exit()
else:
    for i in range(0, n-1, 2):
        if A[i] != i + 1 or A[i] != A[i+1]:
            print(0)
            exit()
M = (max(A) + 1) // 2
print(2 ** M % MOD)

Solution (Python):
n=int(input())
a=sorted(map(int,input().split()))
if (n%2==1 and a==[(i+1)//2*2 for i in range(n)]) or (n%2==0 and a==[i//2*2+1 for i in range(n)]):
    print(pow(2,n//2,10**9+7))
else:
    print(0)

Solution (Python):
import sys


import math
import collections

import bisect
import itertools
import fractions

import copy
import heapq
import decimal

import queue
import numpy as np

sys.setrecursionlimit(10000001)
INF = 10 ** 16
MOD = 10 ** 9 + 7

ni = lambda: int(sys.stdin.readline())
ns = lambda: map(int, sys.stdin.readline().split())
na = lambda: list(map(int, sys.stdin.readline().split()))





def main():
    n = ni()
    a = na()

    c = collections.Counter(a)

    ans = True
    odd = n % 2

    if odd:
        if 0 in c.keys():
            c[0] += n % 2
    for i in range(0 + odd ^ 1, n, 2):
        if i not in c.keys():
            ans = False
            break
        if c[i] != 2:
            ans = False
            break

    if ans:
        print(2 ** (n // 2) % MOD)
    else:
        print(0)


if __name__ == '__main__':
    main()

Solution (Python):
n,*a=map(int,open(0).read().split())
b=[0]*-~n
for i in a:
  b[i]+=1
print(0 if b[0]!=n%2 or max(b)>2 or 1 in b[1:] else pow(2,b.count(2),10**9+7))

Solution (Python):
n,*a=map(int,open(0).read().split())
b=[0]*-~n
for i in a:
  b[i]+=1
print(0 if b[0]!=n%2 or max(b)>2 or 1 in b[1:] else pow(2,b.count(2))%(10**9+7))

Solution (Python):
import sys
from collections import Counter

n = int(input())
arr = list(map(int, input().split()))

if len(arr) % 2 == 0:
    if 0 in arr:
        print(0)
        sys.exit()
else:
    if arr.count(0) != 1:
        print(0)
        sys.exit()
    else:
        arr.remove(0)

cnt = Counter(arr)

values = list(cnt.values())

if values.count(2) != len(values):
    print(0)
    sys.exit()

mod = 10**9 + 7

print(2**(n // 2) % mod)

Solution (Python):
from collections import*
n, *a = map(int, open(0).read().split())
c = Counter(a)
MOD = 10**9+7
if n%2:
    if c.get(0, 0) != 1 or any(c.get(i, 0) != 2 for i in range(2, n, 2)):
        exit(print(0))
    print(2 ** (n // 2) % MOD)
else:
    if any(c.get(i, 0) != 2 for i in range(1, n, 2)):
        exit(print(0))
    print(2 ** (n // 2) % MOD)

Solution (Python):
n = int(input())
A = list(map(int,input().split()))
L = [0]*((n+1)//2)
ans = 1
mod = 10**9+7
for i in A:
    L[i//2] += 1
if n % 2 == 0:
    for i in L:
        if i != 2:
            print(0)
            exit()
        ans *= 2
        ans %= mod
    print(ans)
else:
    if L[0] != 1:
        print(0)
        exit()
    for i in range(1,(n+1)//2):
        if L[i] != 2:
            print(0)
            exit()
        ans *= 2
        ans %= mod
    print(ans)

Solution (Python):
n = int(input())
mod = 10**9+7
L = list(map(int,input().split()))
from collections import Counter
L = Counter(L)

cnt_2 = 0
cnt_0 = 0
else1 = 0
for k,v in L.items():
    if v == 2:
        cnt_2 +=1
    elif k == 0 :
        cnt_0 +=v
    else:
        else1 +=1

if else1 != 0:
    print(0)
elif n%2 == 1 and cnt_0 == 1:
    print(2**cnt_2%mod)
elif n%2 == 1 and cnt_0 != 1:
    print(0)
elif n%2 == 0 and cnt_0 != 0:
    print(0)
elif n%2 == 0 and cnt_0 == 0:
    print(2**cnt_2%mod)

Solution (Python):
import sys

n, *lst = map(int, sys.stdin.read().split())
lst.sort()
mod = 10 ** 9 + 7
if n % 2:
  for i in range(1, n):
    if lst[i] != (i + 1) // 2 * 2:
      print(0)
      quit()
else:
  for i in range(n):
    if lst[i] != i // 2 * 2 + 1:
      print(0)
      quit()
print((1 << (n >> 1)) % mod)

Solution (Python):
from collections import Counter

n = int(input())
A = list(map(int, input().split()))
C = Counter(A)
ans = 1
MOD = 10**9+7
for k, v in C.items():
    if k == 0 and v != 1:
        print(0)
        exit()
    if k != 0 and v != 2:
        print(0)
        exit()
    ans *= v
print(ans % MOD)

Solution (Python):

n = int(input())
a = list(map(int, input().split()))
a.sort()
ans = 1
if n % 2:
    
    if a[0] == 0:
        for i in range(n // 2):
            if a[i * 2 + 1] == a[i * 2 + 1] and a[i * 2 + 1] == i * 2 + 2:
                pass
            else:
                ans = 0
                break
    else:
        ans = 0
else:
    for i in range(n // 2):
        if a[i * 2 + 1] == a[i * 2] and a[i * 2 + 1] == i * 2 + 1:
            pass
        else:
            ans = 0
            break
if ans == 0:
    print(ans)
else:
    print(pow(2, n // 2, 10 ** 9 + 7))