Practice problem · Meta · Hard

Maximum Unique Characters

The system picks a subset of strings to maximize the number of distinct characters covered. 4 stages in Python with an AI assistant. Pro.

Stage 1 of 4: Fix the validation helpers

Background. The system picks a subset of strings to maximize the number of distinct characters covered.

verify.py holds the validation and scoring helpers used by the test suite. The starter has bugs. Tests in tests/test_part1_validation.py fail.

Find it and fix it. Don't change the public interface.

Done when Submit goes green.

Constraints

  • Python 3.12+ standard library only
  • Time limit: 15 seconds per test
  • Editable: picker.py, verify.py
  • Read-only: all tests/test_*.py files and tests/asserts.py
  • Inputs may include any printable characters; the alphabet isn't constrained to lowercase letters

What comes after stage 1

The next stages open once the tests for the one before them pass. Each raises the bar, the way an interviewer does in the room.

  1. Stage 2: Implement the picker.
  2. Stage 3: Optimize for medium inputs.
  3. Stage 4: Scale to the full dataset.

The files you get

  • Yours to edit: picker.py, verify.py.
  • Read-only: tests/__init__.py, tests/asserts.py.
  • Tests you can read and run: tests/test_part1_validation.py. Hidden tests run on Submit.

The round: an agent in the editor

The assistant here works like the one in the interview IDE. It reads your files, runs code and proposes changes, and you accept or reject each one. It doesn't volunteer ideas, so what gets built is what you asked for. Along the way the interviewer asks about your decisions, and at the end the report says what you checked and what you accepted unread.

Frequently asked, briefly answered

Is Maximum Unique Characters free?

It is a Pro problem. Free plan includes one complete problem, plus the first stage of several more, so you can see how a round works before you pay.

Which company asks something like this?

Meta. It is modelled on Meta's round: CoderPad with integrated AI assistant (GPT-4o mini, Claude Sonnet/Haiku, Gemini 2.5 Pro, Llama 4).

Do I bring my own AI tool?

No. The assistant is in the editor, same as in the interview IDEs at Meta, Stripe and Amazon. What you choose at Begin is the format: an agent that proposes changes you accept or reject, an answer-only assistant that can't write code, or no assistant at all.

What does the interviewer ask?

4 questions along the way about your own code, and one at the end about the whole session. A weak answer gets one follow up. The answers go into the report, which ends with a hire or no hire verdict.

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