Data structures
A shop ledger page, 1899. Every line adds up.
LLD Series: LRU / LFU Cache (Low-Level Design)
Phrasings: LeetCode 146/460 but as a class design, Design an in-memory cache like Guava/Caffeine, cache BIN/merchant/config lookups. Asked heavily at Visa, Mastercard, NPCI, Amex, PayU...
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