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The three-pointer pattern is the foundation of every linked list reversal. Understand it once and LC 206, 92, and 25 become variations on the same idea.

Stop casting with `as` and `!`. Discriminated unions give TypeScript enough information to narrow types automatically — making impossible states unrepresentable and exhaustiveness checking free.

TypeScript ships with 15+ utility types that let you derive new types from existing ones. Here's the full toolkit with the production patterns you'll actually reach for.

TypeScript is the default for serious JavaScript work now — but the real question is when the type overhead pays off, and which parts of your codebase genuinely don't need it.

Two pointers, one pass, O(n) — learn how the variable-size sliding window grows and shrinks to solve substring and subarray problems efficiently.

Two matrix search problems, two completely different optimal approaches. Learn when to use virtual 1D binary search vs the staircase elimination — the distinction that trips up most candidates.

LC 297 is a Hard but the insight is simple: null markers make any traversal unambiguous. Learn preorder DFS and BFS serialization, plus the BST shortcut for LC 449.

The Two Sum problem is the "Hello World" of hash map problems. Most people solve it, move on, and treat it as a warmup. That's a mistake. The pattern behind Two Sum — trading space for lookup speed — shows up in dozens of harder problems.

Conditional types and mapped types let you write types that compute — filtering keys, extracting nested types, and deriving new shapes from old ones without duplication.