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Since read4() always reads up to four characters, it may return more characters than required by the current read call. Those extra characters must be saved for future calls to avoid losing data and to maintain correct sequential reading.
Yes, this problem is a common interview question at large tech companies. It tests understanding of API simulation, buffer management, and handling state across multiple function calls.
A small fixed buffer, often implemented as an array with two pointers or indices, works best. It stores leftover characters from read4() that were not used in the previous read operation. This structure allows efficient reuse across multiple calls.
The optimal approach uses a persistent buffer to store extra characters returned by read4(). Each read call first consumes buffered characters before invoking read4() again. This ensures characters are not lost across multiple function calls.