Buggy python submission (verdict: Runtime Error):
import sys

sys.setrecursionlimit(10 ** 5 + 1)

def dfs(i, n, a, L):
    # x + y = n - 1
    # x - y = a
    # x = (n - 1 + a) / 2
    # y = (n - 1 - a) / 2
    if i == n:
        return 1
    if a [i] == 0:
        x = [(n - 1 + a[i]) // 2]
    else:
        x = [(n - 1 + a[i]) // 2, (n - 1 - a[i]) // 2]
    t = 0
    MOD = 10 ** 9 + 7
    for xi in x:
        if L[xi] == 0:
            L[xi] = i + 1
            t += dfs(i + 1, n, a, L)
            L[xi] = 0
    return t % MOD

n = int(input())
a = list(map(int, input().split()))
L = [0 for _ in range(n)]
ans = dfs(0, n, a, L)
print(ans)


Fixed submission:
from collections import defaultdict

MOD = 10 ** 9 + 7
n = int(input())
a = list(map(int, input().split()))
d = defaultdict(int)
for ai in a:
    d[ai] += 1
ans = 1
if n % 2:
    for k, v in d.items():
        if k > 0 and v == 2:
            ans *= 2
        elif k == 0 and v == 1:
            ans *= 1
        else:
            ans = 0
            break
        ans = ans % MOD
else:
    for v in d.values():
        if v == 2:
            ans *= 2
        else:
            ans = 0
            break
        ans = ans % MOD
print(ans)

Buggy python submission (verdict: Runtime Error):
def dfs(i, n, a, L):
    # x + y = n - 1
    # x - y = a
    # x = (n - 1 + a) / 2
    # y = (n - 1 - a) / 2
    if i == n:
        return 1
    if a [i] == 0:
        x = [(n - 1 + a[i]) // 2]
    else:
        x = [(n - 1 + a[i]) // 2, (n - 1 - a[i]) // 2]
    t = 0
    MOD = 10 ** 9 + 7
    for xi in x:
        if L[xi] == 0:
            L[xi] = i + 1
            t += dfs(i + 1, n, a, L)
            L[xi] = 0
    return t % MOD

n = int(input())
a = list(map(int, input().split()))
L = [0 for _ in range(n)]
ans = dfs(0, n, a, L)
print(ans)


Fixed submission:
from collections import defaultdict

MOD = 10 ** 9 + 7
n = int(input())
a = list(map(int, input().split()))
d = defaultdict(int)
for ai in a:
    d[ai] += 1
ans = 1
if n % 2:
    for k, v in d.items():
        if k > 0 and v == 2:
            ans *= 2
        elif k == 0 and v == 1:
            ans *= 1
        else:
            ans = 0
            break
        ans = ans % MOD
else:
    for v in d.values():
        if v == 2:
            ans *= 2
        else:
            ans = 0
            break
        ans = ans % MOD
print(ans)

Buggy python submission (verdict: Runtime Error):
import copy

def dfs(i, n, a, L):
    # x + y = n - 1
    # x - y = a
    # x = (n - 1 + a) / 2
    # y = (n - 1 - a) / 2
    if i == n:
        return 1
    if a [i] == 0:
        x = [(n - 1 + a[i]) // 2]
    else:
        x = [(n - 1 + a[i]) // 2, (n - 1 - a[i]) // 2]
    t = 0
    MOD = 10 ** 9 + 7
    for xi in x:
        if L[xi] == 0:
            Li = copy.copy(L)
            Li[xi] = i + 1
            t += dfs(i + 1, n, a, Li)
    return t % MOD

n = int(input())
a = list(map(int, input().split()))
L = [0 for _ in range(n)]
ans = dfs(0, n, a, L)
print(ans)


Fixed submission:
from collections import defaultdict

MOD = 10 ** 9 + 7
n = int(input())
a = list(map(int, input().split()))
d = defaultdict(int)
for ai in a:
    d[ai] += 1
ans = 1
if n % 2:
    for k, v in d.items():
        if k > 0 and v == 2:
            ans *= 2
        elif k == 0 and v == 1:
            ans *= 1
        else:
            ans = 0
            break
        ans = ans % MOD
else:
    for v in d.values():
        if v == 2:
            ans *= 2
        else:
            ans = 0
            break
        ans = ans % MOD
print(ans)