def best_and_worst_hour(percentages):
    """

    Args:
    percentages: output of compute_percentage
    Return: list of strings, the first element should be the best hour,
    the second (and last) element should be the worst hour. Hour are
    represented by string with format %H:%M
    e.g. ["18:00", "20:00"]

    """
    # Initial setup of variable, so they should be easily swapped.
    bestHour = 100  # <-- init to 100% of labour
    bestHourKey = ""
    worstHour = -1  # <-- init to min lose
    worstHourKey = ""

    for x in percentages:
        value = percentages[x]
        if value > 0:
            if value < bestHour:
                bestHour = value
                bestHourKey = x
        if value < 0:
            if value < worstHour:
                worstHour = value
                worstHourKey = x

    # Creation of list for result variable.
    result = []
    result.append(bestHourKey)
    result.append(worstHourKey)

    return result