Decode Ways: count the decodings of a digit string (1D DP).

Decode Ways is a 1D DP whose difficulty lives entirely in the edge cases: zeros and the valid 1-26 range. The signal is the take-one-digit-or-two recurrence plus disciplined validity checks. Here is the answer.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.