def test_return_string():
string1 = "1,2,3,4,6;5"
clean_list, x = parse_input(string1)
final_list = find_pairs(clean_list, x)
assert return_string(final_list) == '1,4;2,3'
string2 = '2,4,5,6,9,11,15;20'
clean_list, x = parse_input(string2)
final_list = find_pairs(clean_list, x)
assert return_string(final_list) == '5,15;9,11'
string3 = '1,2,3,4;50'
clean_list, x = parse_input(string3)
final_list = find_pairs(clean_list, x)
assert return_string(final_list) == 'NULL'
def test_parse_input():
string1 = "1,2,3,4,6;5"
first, second = parse_input(string1)
assert first == [1, 2, 3, 4, 6]
assert second == 5
def test_number_pairs():
string1 = "1,2,3,4,6;5"
clean_list, x = parse_input(string1)
assert find_pairs(clean_list, x) == [(1, 4), (2, 3)]