Patreon·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jul 2026Remote

Summary

Patreon SWE interview with a Wordle-style coding problem that had two parts. The first part was manageable but the second tripped me up more than I expected for what looked like a simple extension.

Questions Asked (2)

Given a guess string and a target string of equal length, compute a feedback string where each character is G (green, exact match), Y (yellow, letter exists elsewhere in target but not already matched), or W (white, not present). Handle repeated letters correctly so each target letter can only be claimed once.

Algorithms & Data Structures

Author's notes

The green pass first, then yellows thing is the key insight and I actually got there pretty fast.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and edge cases, then propose a two-pass algorithm: first mark greens and count remaining target letters, then mark yellows using the counts while ensuring each target letter is used once. Discuss time and space complexity, and consider optimizations like early termination or using a fixed-size array for counts.

Pro tip: Mention that you can avoid a second pass by using a frequency map and processing yellows in the same pass, but be careful to not double-count letters already matched as green. Also, highlight that the order of marking yellows doesn't matter as long as you respect the counts.

1. Clarify requirements and edge cases

Confirm that guess and target are equal length, discuss handling of repeated letters, and ask about case sensitivity or non-alphabetic characters if relevant.

2. Design the algorithm

Propose a two-pass approach: first pass marks greens and builds a frequency count of unmatched target letters; second pass marks yellows by checking the count and decrementing it.

3. Implement and handle repeats

Use an array or hash map to track remaining counts, ensuring each target letter is claimed at most once. Mark greens first to avoid misclassifying repeated letters.

4. Analyze complexity and test

State that time complexity is O(n) and space O(1) for fixed alphabet. Walk through examples with repeated letters to verify correctness.

5. Discuss optimizations and trade-offs

Mention possible optimizations like using a single pass with careful ordering, or using bitmasks for small alphabets, and discuss trade-offs.

Key Points to Mention

Two-pass algorithm: first mark greens, then yellows using frequency counts.
Use a frequency map or array to track remaining target letters, decrementing when a yellow is assigned.
Handle repeated letters correctly by not reusing a target letter once it's been matched as green or yellow.
Time complexity O(n) and space O(1) for fixed alphabet (e.g., 26 letters).
Edge cases: all greens, all whites, repeated letters in guess and target, empty strings.
Test with examples like guess='crane', target='recap' to illustrate correct handling of repeats.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Extend the evaluator so that after each round, letters confirmed fully absent from the target are tracked, and any future guess containing those letters is flagged as invalid. A letter is only confirmed absent if every instance of it in that guess was marked W.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This is where I underread the problem.

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AI HintsAI Generated

Suggested Approach

First, clarify the data structures and the exact condition for marking a letter as absent. Then, outline the modifications to the evaluator: maintain a set of absent letters, update it after each round based on the guess and feedback, and validate future guesses against this set. Finally, discuss any trade-offs or edge cases.

Pro tip: Emphasize that the absent set should only be updated when a letter is not present in the target at all, which is inferred when all occurrences of that letter in the guess are marked 'W'. This prevents false positives when a letter is present but misplaced.

1. Clarify requirements and assumptions

Confirm the feedback symbols (e.g., 'W' for wrong/absent, 'C' for correct, 'P' for present but wrong position) and the rule for confirming absence. Ask if the target is fixed or if multiple rounds are independent.

2. Design data structures

Propose maintaining a set (or hash set) of absent letters. Also, consider how to track letter occurrences in the guess and feedback to determine absence correctly.

3. Modify evaluator logic

After each round, iterate through the guess and feedback. For each letter, if all its occurrences in the guess are marked 'W', add it to the absent set. Then, before evaluating a new guess, check if any letter is in the absent set; if so, flag the guess as invalid.

4. Handle edge cases and trade-offs

Discuss cases like repeated letters, letters that are absent but not all occurrences marked 'W' (should not be added), and performance implications of set lookups. Consider if the absent set should persist across games or reset.

5. Test and validate

Walk through an example to demonstrate correctness, and mention potential unit tests for the new functionality.

Key Points to Mention

Use a set for O(1) average-time complexity when checking if a letter is absent.
Only add a letter to the absent set if every instance of that letter in the guess received a 'W' feedback.
When validating a new guess, check each letter against the absent set; if any letter is present, the guess is invalid.
Consider the impact on the game loop: invalid guesses might be rejected before evaluation, or flagged with an error.
Discuss how this feature interacts with existing feedback mechanisms (e.g., correct and present letters).
Mention potential optimizations, such as early termination when an absent letter is found in the guess.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.