Final checkpoint: mixed interview rounds
Round three
Two problems, 45 minutes. One is a counting table, one is pure care.
The second problem in this round has no algorithmic difficulty at all and is the one most likely to go wrong. Read it twice, write every rule down as its own line, and write the boundary case beside each rule before you start. That is the whole technique.
Vowel String Limit
Count the lowercase strings of length n that never contain more than k vowels in a row, modulo a large prime. A position on its own cannot express that constraint, so the state needs one more thing. Check n = 0 before you submit.
Refund Ledger Results
Apply a stream of charges and refunds, where a refund goes through only if that specific charge still has enough unrefunded amount left. Report which events applied and the final balance. No clever idea is required. Getting every rule and every boundary right is the entire problem.