Code RoomBase64url decoding
MediumPrep Room Coding #1559

Base64url decoding

CodingSecurityMid–Senior~15 min

Implement base64url decoding (the inverse of JWT-style encoding). Given a base64url string `s` that has had its '=' padding stripped, restore the padding, decode it, and return the original UTF-8 string. The input uses the URL-safe alphabet ('-' and '_'). An empty input decodes to an empty string. Assume the input is well-formed.

Implement
base64url_decode(s: str) → str
Examples
in["aGVsbG8"]out"hello"
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.

0:00 of about 15 min
InputExpectedGot
["aGVsbG8"]"hello"not run yetsample