Prep. Hub

— 15 idioms drilled to muscle memory. Counter, defaultdict, deque, heapq, bisect, slicing, comprehensions. Language fluency first — every pattern below assumes you can type these in 30 seconds.

— Dense single-page map of every pattern. The "what exists" overview. Print landscape A4 and keep beside you while you code.

— Signal-to-pattern cards. Click to flip. Mark as known. Filter by category. Turn the cheatsheet's passive map into a recognition reflex. Each example links to its practice script.

— Step through the dedup invariant side by side: WITHOUT skip vs WITH skip. Watch the duplicate triplet appear in one panel and not the other.

— Watch the window grow on the right and shrink on the left for Longest Substring Without Repeating. Both pointers move forward — see why.

— Four modes of the same skeleton: exact match, insertion point, first occurrence, last occurrence. Step through and watch which return changes and why.

— Push, pop, and peek visualized. Two scenes: Valid Parentheses matching, and Min Stack with its auxiliary stack synced in parallel.

— Watch the hash map and doubly-linked list stay in sync through put / get / evict. See exactly why move_to_end and popitem are O(1).

— Sliding window with the swap-budget check size − max_count ≤ k. Watch L and R fingers walk the string and the counts table stay in sync.

— Monotonic stack: each day waits in line for a warmer one. Watch indices push, pop, and fill the result array as warmer days arrive.