Google·Software Engineer·Onsite - Coding / Algorithms·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Google SWE coding round focused on string parsing and stack-based decoding. The problem itself wasn't too bad but the variants they throw at you are where things get interesting.

Questions Asked (1)

Given an encoded string like '3[a2[c]]', write a function to decode it by expanding each bracketed group by its repeat count.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I went straight for the recursive approach because it felt cleaner to think about, but partway through they pushed me toward an iterative stack solution.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases, then propose a stack-based solution that processes the string character by character, using stacks to track repeat counts and partial results. Walk through the example to demonstrate correctness, and discuss time and space complexity.

Pro tip: Mention that you would handle multi-digit numbers and nested brackets, and that you'd consider using recursion as an alternative but prefer iteration to avoid stack overflow for deeply nested inputs.

1. Clarify requirements and edge cases

Ask about input constraints (e.g., valid encoded strings, max length, digit counts, nesting depth) and expected output for edge cases like empty strings or no brackets.

2. Outline the stack-based approach

Explain that you'll use two stacks: one for repeat counts and one for strings built so far. Iterate through the string, pushing counts when encountering digits and pushing current string when encountering '['.

3. Detail the decoding logic

When encountering a letter, append to current string; when encountering ']', pop a count and a previous string, then append the current string repeated count times to the previous string and set it as current.

4. Walk through an example

Trace the algorithm on '3[a2[c]]' to show how the stacks evolve and produce 'accaccacc', ensuring the interviewer follows your reasoning.

5. Analyze complexity and trade-offs

State that time complexity is O(n * maxK) where n is output length and maxK is maximum repeat count, and space is O(n). Discuss recursion vs iteration and potential optimizations.

Key Points to Mention

Use of two stacks to handle nested structures and multi-digit numbers.
Handling of multi-digit repeat counts (e.g., '12[a]') by accumulating digits.
Time and space complexity analysis, noting that output size can be exponential in nesting depth.
Edge cases: empty string, no brackets, unbalanced brackets (if input may be invalid).
Alternative recursive approach and its trade-offs (simpler code but risk of stack overflow).
Potential optimizations like using a single stack of pairs or building the result with a StringBuilder.

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