Not Sure What Is Wrong With My Solution For 264

Ultimately it gave the wrong answer for the last test case: ULVYWZXGRCSMDEFBJHIKNTOPQA 41814627759

Expected was ULUYUZVGRCSMDDDBJGGJDSKKPA, but my program produced ULUYUYUGRCRLDEDBJHEEBRFDCU

But... I don't even see how the answer I produced was wrong. For the last present in the wishlist, there are 26 possibilities, and the choice for each character in the wishlist is independent of the others (I believe), and 41814627759 % 26 = 21, U is the 21st letter of the alphabet... so how could the answer possibly have 'A' as the last character?

Is Z the 26th letter of the alphabet? If it is, would your expression (N % 26) ever return Z?

Okay okay, you got me there. I gloss over the fact that my solution actually computes it zero-indexed, so I pass in 1 less than the original input. So really my program would end up doing 41814627758 % 26 = 20, and U is the 20th letter of the alphabet if A is the 0th. With that, Z is a possible result of such a calculation.

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Hello reader of this forum thread. I've redacted what I've written here to not spoil you with the approach that I took for this problem.

Great. Now all I suggest is checking the off-by-one discrepancy in the sixth letter. Feel free to display N % 26 at each iteration as well, so you can check that it is not, as a matter of fact, an invariant.

My last comment was made before looking at this comment, pondering it

Feel free to display N % 26

To be clear, I wasn't saying that I'm treating things as a ▮▮▮▮▮▮▮▮▮▮▮▮▮▮(another redaction forum reader, sorry). N % 26 only matter for the last character, because there are 26 choices for the last character.

Now all I suggest is checking the off-by-one discrepancy in the sixth letter

I'm thinking about it right now, but at least off the top of my head, I don't see how there could be an off-by-one there, because I start computing the result from right to left. Earlier errors(which happen rightwards in the result string) would accumulate.

That is very interesting

I'm not sure to interpret that as meaning it is an approach that differs from yours, or if it means that you are trying to tell me that I should be computing it from left to right.

Oh, also: "because there are 26 choices for the last character." Are there?

There are 26 presents. So the last child doesn't get any say in what present they get, because there's only one present left. So they could have wished for any of the 26 presents and they would have gotten the same thing in the end.

My program works for all of the test-cases except the last one... which has a length of 26. I realize what it is about 26 that makes my program treat it differently, but with my current understanding of the problem (which is evidently wrong), I would believe that actually the length 26 case is the only case in which my program is correct, and all the other ones are wrong.

I've employed a hacky modification to the program that makes it work for sizes <= 26. Submitted it, and it passed.

I don't understand at all. I need to reevaluate a fundamental part of the problem. I'll go through smaller test cases with a brute-force program and think about why what I thought was in fact wrong.