CodingMediumcod-g607

Subject RecursionLevel Mid–Senior~25 minCommon in Algorithms & data structures interviewsIndustries Software development

Question

A message of digits is encoded by mapping 'A'->1, 'B'->2, ..., 'Z'->26. Given a non-empty digit string, return the number of ways to decode it back into letters. A valid single digit is 1-9 and a valid two-digit group is 10-26; a leading '0' in a group is invalid. The string length is at most 100.

Implement

num_decodings(s: str) → int

Examples

in["226"]out3

What a strong answer looks like

State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.

Learn the concepts

Vibe coding: describe the solution in plain language (or narrate it) and the coach grades your approach. Generating runnable code from your description is coming next.

Run or narrate your approach, then ask the coach.