Reverse Binary String
Reported by candidates from Amazon's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Amazon asked this in March 2024, and it's a two-pointer problem that looks deceptively simple until you hit the edge cases. You're reversing a binary string, which sounds straightforward, but the catch is usually in how you handle leading zeros or the efficiency requirement. If you blank on the approach during the live OA, StealthCoder will surface the two-pointer pattern instantly so you don't lose momentum.
Pattern and pitfall
The naive approach is to convert to a list and swap from both ends, but Amazon cares about whether you think about space and time. Two pointers starting at index 0 and length minus 1 means you swap in O(n) time and O(1) space if you're using a language that lets you mutate strings directly. The real trick is recognizing that binary strings don't need special handling for leading zeros like decimal numbers do, so you just reverse the whole thing. Common pitfall: overthinking whether zeros matter or trying to reverse without actually swapping. When you're live and feel stuck, StealthCoder gives you the two-pointer template so you can execute cleanly.
If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.
You can drill Reverse Binary String cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it.
Get StealthCoderRelated leaked OAs
This OA pattern shows up on LeetCode as reverse string. If you have time before the OA, drill that.
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Amazon reuses patterns across OAs. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Reverse Binary String FAQ
Is this really a two-pointer problem or is there a trick?+
It is two-pointer. No trick. Start at both ends and swap characters until pointers meet. The gotcha is that some candidates try to do it in one pass or use recursion when the iterative two-pointer solution is what Amazon wants.
Do I need to worry about leading zeros in a binary string?+
No. Binary strings don't have the semantic leading-zero issue that decimal numbers do. Reverse the whole string without special cases. If the problem explicitly forbids leading zeros in the output, you'd handle it, but that's not typical for this version.
Should I use a built-in reverse method or code it myself?+
Always code it yourself in an interview. Built-in methods feel like you're dodging the question. Two pointers is the pattern Amazon is testing for. Show that you know how to iterate and swap.
What's the space complexity Amazon expects?+
O(1) if the language allows in-place string mutation. O(n) if you have to create a new string. Mention both possibilities. In Python, strings are immutable, so you'd convert to a list first, then convert back. Amazon accepts that trade-off.
Can I solve this in 2-3 minutes or is there a hidden depth?+
Genuinely 2-3 minutes if you know two pointers. The problem is not a complexity trap. It's a pattern check. If you're spending more than 5 minutes coding, you're overcomplicating it.