Dynamic Programming — Decode Ways

A message of letters A–Z was encoded as numbers: 'A' → '1',
'B' → '2', ..., 'Z' → '26'. Given the digit string digits, return
how many ways it can be decoded.

'12' is 'AB' or 'L': 2 ways. A '0' can never stand alone, and a
leading zero like '06' is not a valid code, so zeros are where most attempts
break.

ways[i] (ways to decode the first i digits) comes from at most two places:
the last digit alone, if it is 1–9, adds ways[i - 1]; the last two
digits together, if they form 10–26, add ways[i - 2].

Examples

solve('12')   → 2   // 'AB', 'L'
solve('226')  → 3   // 'BZ', 'VF', 'BBF'
solve('06')   → 0

Constraints

• 1 <= digits.length <= 100, digits only
• The answer fits in a 32-bit integer.