The Read N Characters Given read4 II problem extends Part I: the read function may be called multiple times and must maintain state between calls (a leftover internal buffer from previous read4 calls). This hard design problem requires persisting state across function invocations. The array, interactive, and simulation interview pattern is demonstrated at an advanced level.
Apple, Lyft, Meta, Google, and Bloomberg ask this hard variant because it tests stateful iterator design — maintaining an internal buffer and position across multiple calls, a common pattern in stream readers, parsers, and network I/O.
Internal buffer with state persistence. Maintain class-level variables: buf4 (internal 4-char buffer), buf4_ptr (pointer into buf4), buf4_count (valid chars in buf4). For each read(buf, n) call: first drain from buf4 if any leftover chars. Then call read4 as needed, keeping leftover chars in buf4 for next call.
File="abcde". read(buf, 1): buf4=["a","b","c","d"], buf4_ptr=0, buf4_count=4. Copy 1 char. buf="a". buf4_ptr=1. State: 3 chars remaining. read(buf, 2): drain from buf4. Copy 2 chars "bc". buf="bc". buf4_ptr=3. State: 1 char remaining. read(buf, 3): drain 1 from buf4 ("d"), then read4 gets "e"(1 char). Copy 1+1=2 chars. buf="de". Return 2.
Read N Characters II is a stateful buffer management problem. The critical difference from Part I: leftover characters from read4 must be cached in an instance variable, not discarded. This models real-world I/O buffering. Practice implementing stateful iterators: "flatten nested iterator," "buffered file reader," "chunked protocol parser." Each requires careful state management between consecutive calls.
