Alphanumeric Combinations

Problem Overview

• Count the number of valid decodings of a digit string where 1–26 map to letters a–z.
• Input: a digit string S (0–9); Output: a single integer count of possible words; return 0 if none.
• Context: models number-to-letter learning and alphanumeric combinations in English words.
• A classic coding interview problem and interview question asked in Goldman interviews.