Find the first missing element between 1 and N in an array of integers. -1000000 <= x <= 1000000

find_first_missing.py

LIMIT = 1000000


def unmaskNumber(number):
    if number <= -LIMIT:
        number = number + LIMIT
    elif number > LIMIT:
        number = number - LIMIT
    
    return number


def maskNumber(number):
    if number <= 0:
        number -= LIMIT
    else:
        number += LIMIT
    
    return number


def solution(A):
    for index in range(len(A)):
        number = unmaskNumber(A[index])
            
        if number <= 0:
            continue
        if number > len(A):
            continue
        
        A[number - 1] = maskNumber(A[number - 1])
  
    for index in range(1, len(A)):
        if A[index] > -LIMIT and A[index] < LIMIT:
            return index + 1

    return len(A) + 1


print(solution([0, 1, 2, 3]))
print(solution([1, 2, 3]))
print(solution([6, -2, 1, 3, 8]))