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Hand-picked problems across 6 languages — banking, e-commerce, analytics, system design. Run them right in your browser; ask AI when stuck.
Two Sum (Banking)
You're auditing a set of transaction amounts and need to find two transactions that together reconcile to exactly `target`. Return the 0-based indices of the two transactions. Exactly one solution exists.
Reverse a String
Reverse a given string without using Python's built-in slice `[::-1]` — demonstrate the two-pointer technique.
Group E-Commerce Orders by Customer
You're given `n` order lines from an online store. Each line has a customer ID and an amount. Compute the total revenue per customer and print customers in ascending order of ID.
Build an LRU Cache
Implement an LRU (Least Recently Used) cache with O(1) `get` and `put`. Used by every web browser, CDN, and database query planner.
Longest Substring Without Repeating
Given a string, return the length of the longest substring with all distinct characters. Classic sliding-window problem.