def merge(a, b):
    """Merges two sorted lists"""
    i, j = 0, 0
    r = []
    while i < len(a) and j < len(b):
        if a[i] < b[j]:
            r.append(a[i])
            i += 1
        else:
            r.append(b[j])
            j += 1

    while i < len(a):
        r.append(a[i])
        i += 1

    while j < len(b):
        r.append(b[j])
        j += 1
    return r


def merge_sort(v):
    length = len(v)
    if length <= 1:
        return v

    half_length = length // 2

    a = merge_sort(v[:half_length])
    b = merge_sort(v[half_length:])
    return merge(a, b)


if __name__ == "__main__":
    print(merge_sort([42, 12, 3, 1, 5]))