Linh Ha (haconglinh1990@gmail.com) — Solutions for Problems 1, 2, 3

[haconglinh1990]

Submission by Linh Ha (@haconglinh1990) — three solved problems.

What's included

• Problem 1 — src/problem1/sum_to_n.js
  Three implementations of sum_to_n(n): iterative O(n), closed-form Gauss O(1), and a functional reduce O(n)/O(n). Each has complexity notes and handles negative n symmetrically. A small self-check runs when invoked as node sum_to_n.js.

• Problem 2 — src/problem2/
  A polished currency swap form built with Vite + React + TypeScript + Tailwind + Headless UI + framer-motion + sonner. Dark glassmorphism aesthetic. Live exchange rates from https://interview.switcheo.com/prices.json (fetched once, deduped to latest entry per symbol, tokens without prices dropped). Searchable token picker, two-way amount binding, USD value display, flip-direction button, inline + on-blur validation, disabled submit with tooltip reason, mock 1.5 s submit with a success toast, loading skeleton and error-retry states. 61 unit tests cover the pure logic and the data hook. prefers-reduced-motion respected.

• Problem 3 — src/problem3/
  Refactor of the buggy WalletPage. Original.tsx preserves the original verbatim for reference; README.md documents six correctness bugs plus a list of TypeScript and performance issues; WalletPage.tsx is the cleaned-up version with a hoisted priority lookup, decorate-sort-undecorate, split filter/format memos with correct dependency arrays, and stable composite keys.

Problems 4 and 5 are not attempted.

How to verify

Problem 1:

node code-challenge/src/problem1/sum_to_n.js

Problem 2:

cd code-challenge/src/problem2
npm install
npm run dev      # open the printed URL
npm test         # 61/61 pure-logic tests
npm run build    # type-check + production build

A smoke-test walkthrough is in src/problem2/README.md.

Problem 3:
The refactor compiles in isolation only if the original project hooks (useWalletBalances, usePrices, WalletRow, BoxProps, classes) are wired in — see the comment block at the top of WalletPage.tsx. README.md walks through the issues one by one.

Notes on the spec/plan trail

Design spec and step-by-step implementation plan for Problem 2 are committed under docs/superpowers/specs/ and docs/superpowers/plans/ so the reasoning behind the architecture and validation rules is auditable.