Read N Characters Given read4 II - Call Multiple Times
A hard-tier problem at 43% community acceptance, tagged with Array, Simulation, Interactive. Reported in interviews at Lyft and 1 others.
Read N Characters Given read4 II is the harder sibling of a classic problem asked at Google and Lyft. You're building a file reader that calls a limited API (read4) multiple times, buffering partial reads across calls. The 42% acceptance rate isn't just about the algorithm; it's about state management. Most candidates nail the first call, then blank when the second call arrives and the buffer's half-full. If this hits your live OA and you haven't drilled the stateful buffer pattern, StealthCoder solves it invisibly in seconds.
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Read N Characters Given read4 II - Call Multiple Times is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an Amazon engineer who used it to pass JPMorgan's OA and system design loop.
Get StealthCoderThe trick here isn't algorithmic complexity; it's bookkeeping. read4 returns up to 4 characters from a file. You need to read exactly N characters across multiple function calls, keeping state between calls. The common failure: forgetting to buffer leftover characters from the previous read4 call, or losing track of position in that buffer. You'll implement a class that maintains queue-like state (what characters are cached, what index into the cache you're at, whether the file's exhausted). Most candidates who've only seen part one treat each call as independent, which fails immediately on the second invocation. StealthCoder acts as your failsafe if the multi-call statefulness trips you up during the real assessment.
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Read N Characters Given read4 II - Call Multiple Times recycles across companies for a reason. It's hard-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an Amazon engineer who used it to pass JPMorgan's OA and system design loop. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Read N Characters Given read4 II - Call Multiple Times interview FAQ
Why is the acceptance rate so low for this problem?+
Stateful bookkeeping across multiple calls is harder than it looks. Candidates often handle the first call correctly but fail on subsequent calls because they don't properly buffer characters or track their position in the internal buffer. Off-by-one errors in the cache are common.
Does this problem actually appear at Google and Lyft?+
Yes. Both companies include it in their assessment flows. It's especially common in roles where you're building internal APIs or file-handling utilities. The multi-call aspect makes it more realistic than the single-call version.
What's the main difference from Read N Characters Given read4 I?+
Part I is a single function call. Part II requires you to implement a class that remembers state between multiple calls to the same method. You're essentially simulating a stateful file pointer that persists across invocations.
How do I avoid the buffer state pitfall?+
Use a queue or cache to store the last read4 result. Track the index into that cache and refill it only when depleted. Clearly separate the logic for 'read from cache' vs 'call read4 and populate cache'. Test with at least three consecutive calls.
Is this considered an interactive problem?+
Yes, because your solution interacts with an external read4 function you don't control. You're adapting your code to an API contract, not just manipulating data. That's why the problem is tagged Interactive.
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