Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Instacart software engineer interview with a grid traversal problem that sounds straightforward until you actually sit down with it. The edge cases pile up fast and I spent way too long second-guessing the error handling.

Questions Asked (1)

You're given a 2D character grid, a starting position, and a sequence of move directions (up, down, left, right). Traverse the grid following the moves and build a password string by appending each visited cell's character, but skip duplicates of the last appended character. Cells marked '#' are blocked. If any move goes out of bounds or into a blocked cell, return an error immediately. What's your approach and what are the time and space complexities?

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I started with the traversal logic and that part felt fine, just track position and simulate each move.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and edge cases, then outline a step-by-step simulation approach that tracks the current position and builds the password while checking for invalid moves. Finally, analyze the time and space complexities, emphasizing that the algorithm is O(n) time and O(1) extra space (excluding output).

Pro tip: Mention that you would validate the input grid and moves upfront, and discuss how to handle the error return (e.g., throwing an exception or returning a sentinel value) based on the interviewer's preference.

1. Clarify requirements and edge cases

Ask about grid dimensions, starting position validity, move sequence length, and what 'return an error' means (e.g., exception, null, or specific string). Confirm that '#' cells are blocked and that duplicates are only skipped if they match the last appended character.

2. Design the simulation algorithm

Initialize the current position and an empty password string. For each move, compute the new position, check if it's within bounds and not blocked; if invalid, return an error. Otherwise, update position and append the cell's character if it differs from the last appended character.

3. Analyze time and space complexity

Time complexity is O(n) where n is the number of moves, as each move is processed in constant time. Space complexity is O(1) extra space (excluding the output string), since only a few variables are used.

4. Discuss potential optimizations and trade-offs

Consider if the grid is very large or moves are numerous; the simulation is optimal. Mention that early termination on invalid move saves time. If the output string is large, note that space is O(m) where m is the length of the password.

Key Points to Mention

Boundary checking and blocked cell detection before each move
Condition for appending character: only if different from last appended
Error handling: immediate return on invalid move
Time complexity O(n) for n moves
Space complexity O(1) extra space, O(m) for output string
Input validation and edge cases (e.g., empty moves, starting on blocked cell)

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