from functools import reduce

list_in = [[], [67, 76, 89], [1, 12], [2, 4, 6, 8, 10, 11], [3, 5, 7]]

def sort(list_1, list_2):
    index_1 = 0
    index_2 = 0
    list_out = []
    while True:
        if index_1 == len(list_1):
            list_out.extend(list_2[index_2:])
            break
        if index_2 == len(list_2):
            list_out.extend(list_1[index_1:])
            break
        if list_1[index_1] <= list_2[index_2]:
            list_out.append(list_1[index_1])
            index_1 += 1
        else:
            list_out.append(list_2[index_2])
            index_2 += 1
    return list_out

print(list(reduce(sort, list_in)))