missing_numbers.py

def missing_numbers(numbers: list) -> str:
    """
    Returns a comma-separated number/range of numbers which are missing in the specified list
    # O(n) solution; n - size of the input list of numbers
    """

    i = 0
    lnumbers = len(numbers)

    result = ''
    while(i < lnumbers - 1):
        if numbers[i] + 1 != numbers[i+1]:
            start = str(numbers[i] + 1)
            end = str(numbers[i+1] - 1)
            if start == end:
                # single missing number
                end = ''
            result += start
            if end:
                # end of a range of missing numbers
                result += '-' + end
            result += ','
        i += 1

    return result.rstrip(',')


assert missing_numbers([1, 2, 4, 5, 49]) == '3,6-48'
assert missing_numbers([1, 2, 3, 4]) == ''
assert missing_numbers([10, 20, 30, 40]) == '11-19,21-29,31-39'
assert missing_numbers([5, 10, 11]) == '6-9'
assert missing_numbers([5]) == ''