Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Interviewed for a software engineer role at Instacart and got a string decoding problem involving nested bracket patterns. Pretty classic stack-based question but the nesting part is where things get interesting.

Questions Asked (1)

Implement a function to decode an encoded string where a number followed by a bracketed substring means that substring repeated that many times. Patterns can be nested inside each other.

Algorithms & Data Structures

Author's notes

The simple cases like '3[a]2[bc]' are easy enough to reason through, but the nested version like '3[a2[c]]' is where I had to slow down and actually think about the stack structure.

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AI HintsAI Generated

Suggested Approach

Use a stack-based approach to handle nested patterns: iterate through the string, pushing characters and numbers onto a stack until a closing bracket is encountered, then pop and repeat the substring accordingly. Alternatively, use recursion to parse the string, treating each bracketed section as a recursive call. Both methods efficiently handle nesting and repetition.

Pro tip: Clarify edge cases upfront, such as multi-digit numbers, empty brackets, and invalid input, to demonstrate thoroughness. Also, discuss time and space complexity: O(n) time where n is the length of the decoded string, and O(m) space where m is the depth of nesting.

1. Understand the problem and clarify constraints

Ask about input format, character set, maximum nesting depth, and whether the input is guaranteed valid. Confirm expected output and edge cases like empty strings or numbers with multiple digits.

2. Choose an approach: stack or recursion

Decide between iterative stack-based parsing or recursive descent. Explain the trade-offs: stack avoids recursion depth limits, while recursion can be more intuitive for nested structures.

3. Outline the algorithm

For stack: traverse the string, push characters and numbers onto a stack; on ']', pop until '[', then pop the number, repeat the substring, and push back. For recursion: parse until ']', building substrings and repeating when a number precedes '['.

4. Implement and test with examples

Write clean code with helper functions if needed. Test with simple cases like '3[a]', nested cases like '3[a2[c]]', and edge cases like '10[ab]' or empty input.

5. Analyze complexity and optimize

Discuss time and space complexity. Consider optimizations like using a StringBuilder for efficient string concatenation or avoiding unnecessary string copies.

Key Points to Mention

Stack-based parsing for nested structures
Handling multi-digit numbers (e.g., '12[a]')
Recursion as an alternative with base cases
Time complexity: O(n) where n is the length of the decoded string
Space complexity: O(m) where m is the maximum nesting depth
Edge cases: empty brackets, invalid input, large repetition counts

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