Thank you for your comment. I think you're right, so i refactored my solution.

The task here is to decipher the task. :D

String str = "abc";

is equivalent to:

char data[] = {'a', 'b', 'c'};

(http://docs.oracle.com/javase/8/docs/api/java/lang/String.html)

imho it's always a good idea to be prepared
(you will often see here on codewars, that the visible test-cases not always cover all posible incidents)

once again...

date_nb_days(100, 150, 2.00) --> "2035-12-26" (7299 days)

from 2016-01-01 to 2035-01-01 = 19 years, right?
from 2016-01-01 to 2036-01-01 = 20 years, right?
from 2016-01-01 to 2035-12-26 < 20 years, right?
years | sum | interest
00 | 100,000 | 0,000
01 | 102,000 | 2,000
02 | 104,040 | 2,040
03 | 106,121 | 2,081
04 | 108,243 | 2,122
05 | 110,408 | 2,165
06 | 112,616 | 2,208
07 | 114,869 | 2,252
08 | 117,166 | 2,297
09 | 119,509 | 2,343
10 | 121,899 | 2,390
11 | 124,337 | 2,438
12 | 126,824 | 2,487
13 | 129,361 | 2,536
14 | 131,948 | 2,587
15 | 134,587 | 2,639
16 | 137,279 | 2,692
17 | 140,024 | 2,746
18 | 142,825 | 2,800
19 | 145,681 | 2,856
20 | 148,595 | 2,914
21 | 151,567 | 2,972

to get 150 it needs more than 20 years
can someone pls explain whats wrong with my calculation?

so until that you compound annually
and then daily...

when I compound only daily, it's the same result:
interestsPerDay = 100 * 0,0098 / 360 = 0,00272222222222222222222222222222
days = 1 / 0,00272222222222222222222222222222 = 367,34693877551020408163265306152
why is the answer 366 days?

Hello, I'm confused... and can't find my fault - please help!

As an example you give "date_nb_days(100, 101, 0.98) --> "2017-01-01" (366 days)".
After a year: 100 -> 100,98 (100 * 1,0098 or 100 + 100 * 0,98 / 100 ) and 360 days.
InterestsPerDay now: 100,98 * 0,98 / 36000 = 0,0027489.
days / sum
361 / 100,9827489
362 / 100,9854978
363 / 100,9882467
364 / 100,9909956
365 / 100,9937445
366 / 100,9964934
367 / 100,9992423
368 / 101,0019912

other try:
0,02 / 0,0027489 = 7,2756375277383680745025282840409

Can you please explain, how you came to 366 days?

Dear g964, not a problem, but inconsistency.
in your examples:
"[["M: 37", "B: 5", "C: 4"], ["M: 100", "B: 14", "C: 11"]]"
in test cases:
"[[M: 37 B: 5 C: 4][M: 100 B: 14 C: 11]]"

Thank you for that kata. It was fun to solve it ;]

look for the difference in these two lines:

[[M: 37 B: 5 C: 4][][M: 100 B: 14 C: 11]...
[[M: 37 B: 5 C: 4][, ][M: 100 B: 14 C: 11]...

Thx shaurya157!

Hello WilliamWu, i tested your code and it's incomplete!
It only runs if m < n.

Pls look at my solution and see "the problem".
I dont know how to "fix it".

Thank you very much for this kata.
It was a nice challenge, with a lot of learning effects and fun :-D