Software Engineer Interview Experience Decoding Encoded Strings
Question Details
Got asked this question for an interview:
```
""" Given an encoded string,
return its decoded string. The encoding rule is: k[encodedstring], where the encodedstring inside the square brackets is bein
Full Details
Got asked this question for an interview:
""" Given an encoded string,
**return** its decoded string. The encoding rule is: k[encodedstring], where the encodedstring inside the square brackets is being repeated exactly k times.
**Note** that k is guaranteed to be a positive integer. ex:
**Example 1**:
**Input**: s = "3[a]2[bc]"
**Output**: "aaabcbc"
**Example 2**:
**Input**: s = "3[a2[c]]"
**Output**: "accaccacc"
**Example 3**:
**Input**: s = "2[abc]3[cd]ef"
**Output**: "abcabccdcdcdef" """
Since I was used to standard OOP/OOD for my interviews, got a bit caught off guard that this problem seemed like a leetcode problem. So I ended up needing to ask clarifying questions about the requirements and reason from first principles ## What I did * Clarify if the encoded string to decode will be a valid input? Luckily we don't need to handle that * Pick a test case that is small but generalizable. Some good examples already provided * Walk through how I would do the task as a human. Certain patterns emerged which led to a recursive approach ## Implementation * I was able to get some of the boilerplate interfaces scoped * Carefully went through edge cases as I was scanning the string to build the decoding * Had the most trouble with how to persist the state of the decoded string when doing the recursive part and combining with the current stack's output * Minor but subtle bug took time for me to reason about ## What was the outcome * I was just in the nick of time able to get a working implementation that passes the provided test cases * Interviewer needed to provide some hints in terms of debugging the recursion implementation. We ran through the debugging like a pair programming exercise * Recruiter followed up and said that I
passed. Interviewer really liked how I "systematically thought through the edge cases in my implementation" Key takeaway is that it's not over until it's over. You might think you "bombed the interview", but maybe you actually
passed. Little things like being systematic on your edge case walkthrough, asking clarifying questions, good interface design do shift the outcome despite not 100% solving all the parts in the allotted time
About Square/Block Interview Reports
This question was reported by a candidate who interviewed at Square/Block. LeakCode aggregates interview reports from 10+ sources, including 1Point3Acres, Glassdoor, LeetCode Discuss, Blind, Reddit, Indeed, and Nowcoder. Each report is translated where necessary, deduplicated against existing entries, and tagged by company, role, round type, and reporting date.
Use this question as one calibration data point, not a memorization target. Companies typically rotate their question pools every 2-4 months; the exact wording of a 2024 question may differ from what you encounter today. The underlying pattern, difficulty level, and follow-up depth at Square/Block are the higher-signal extractions to take from this report.
For broader preparation context, the Square/Block interview process typically includes a recruiter screen, one or two technical phone screens, and a 4-5 round on-site loop covering coding, system design (at L4+ levels), and behavioral. Reports tagged on LeakCode show the round-by-round distribution and typical difficulty calibration. To browse questions filtered by round type and seniority, use the company hub linked above.
How To Practice This Type of Question
Solve similar problems on LeetCode under timed conditions (25-35 minutes per medium difficulty). The goal is pattern recognition: recognize the underlying technique (sliding window, two-pointer, BFS, memoized recursion, etc.) within 60-90 seconds of reading. Strong candidates verbalize their hypothesis out loud before coding, then iterate based on feedback. Weak candidates dive into implementation immediately, lose time on the wrong approach, and run out of time for follow-ups.
Companies update their question pools every 2-4 months. The exact wording of any given question may have been retired by the time you interview. Focus your prep on the pattern, not the specific problem. The patterns that appear in Square/Block reports consistently are the ones worth investing in; one-off niche problems are not.
During Your Square/Block Round
Apply the standard interview round template: clarify requirements (2-3 minutes), state your approach out loud and confirm direction with the interviewer (3-5 minutes), code with narration (15-25 minutes), test with concrete examples including edge cases (5 minutes), discuss optimization or trade-offs if time permits (5 minutes). This template is universally accepted across FAANG and adjacent companies; deviating from it produces weaker interviewer feedback signal.
The single most predictive failure mode in Square/Block reports tagged "no hire": not asking clarifying questions. Interviewers are explicitly trained to weight this. Strong candidates ask 3-5 clarifying questions even on problems that look obvious; weak candidates dive into code immediately. The clarifying-question check is often the first signal recorded in the interviewer's written notes.