Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Coding screen for a software engineer role at Instacart. The problem looked like a file parsing puzzle at first glance but kept growing in scope as the interviewer added constraints.

Questions Asked (1)

You're given a file path to a text file. The file has an index line like [x, y] followed by a grid of characters where the origin is at the bottom-left. Return the character at coordinate (x, y). Then extend your solution to handle multiple index-matrix blocks in the same file: read each character, prepend it to a password string based on its index order, and return the password. If there are multiple passwords in the file, stop after completing the first one.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The first part wasn't bad, just coordinate lookup with some off-by-one anxiety around the lower-left origin.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the input format and coordinate system, then outline a two-phase solution: first parse a single block and return the character at (x, y), then extend to multiple blocks by reading each block, extracting the indexed character, and prepending it to build the password. Emphasize edge cases like invalid coordinates, multiple passwords, and stopping after the first complete password.

Pro tip: Demonstrate maturity by discussing trade-offs between parsing the entire file upfront versus streaming block-by-block, and mention how you'd handle malformed input or ambiguous block boundaries.

1. Clarify requirements and assumptions

Ask about file format details: how blocks are separated, whether indices are 0-based or 1-based, and what 'prepend based on index order' means. Confirm that the origin is bottom-left and that y increases upward.

2. Design single-block solution

Parse the index line to get (x, y), read the grid lines, and map coordinates: since origin is bottom-left, the character at (x, y) is at grid row (height - 1 - y) and column x. Return that character.

3. Extend to multiple blocks

Iterate through the file, detecting each index line and its following grid. For each block, extract the character at the given coordinate and prepend it to a password string. Stop when a password is complete (e.g., when the next block starts or file ends).

4. Handle edge cases and validation

Check for out-of-bounds coordinates, empty grids, missing index lines, and multiple passwords. Decide how to signal errors (e.g., return empty string or raise exception) and ensure you stop after the first password.

5. Analyze complexity and trade-offs

Discuss time and space complexity: O(N) for reading the file, O(1) extra space if streaming. Mention trade-offs between parsing all blocks at once versus streaming, and how to handle large files efficiently.

Key Points to Mention

Coordinate system transformation: bottom-left origin means y-axis inversion when mapping to array indices.
Parsing strategy: how to detect block boundaries (e.g., lines starting with '[') and handle varying grid sizes.
Password construction: prepending characters in the order blocks appear, and stopping after the first complete password.
Edge cases: invalid coordinates, malformed blocks, multiple passwords, and empty files.
Complexity analysis: O(N) time and O(1) or O(k) space depending on streaming vs. storing blocks.
Trade-offs: streaming vs. loading entire file, and robustness vs. simplicity in parsing.

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