[FE] Pahmi Alifya Bahri - pahmi1998@gmail.com - #299

Summary

Solutions for the 99Tech code challenge, applying for the Frontend Engineer
role. Problems 1–3 are attempted (Problems 4–5 are out of scope for this role
and left untouched).

• Problem 1 — three sum_to_n implementations (loop, closed-form formula,
  recursion) plus a runTestCases() helper that checks all three against a
  shared set of inputs.
• Problem 2 — a currency swap form (React + TypeScript + Vite) using live
  prices and real token icons, with input validation and a simulated submit
  flow.
• Problem 3 — a written audit of the provided "Messy React" snippet's
  bugs/inefficiencies, plus a refactored version.

Problem 1 — Three ways to sum to n

src/problem1/sum_to_n.js

• sum_to_n_a: iterative loop — O(n) time, O(1) space.
• sum_to_n_b: Gauss' formula n*(n+1)/2 — O(1) time, O(1) space.
• sum_to_n_c: recursion — O(n) time, O(n) call-stack space.
• runTestCases() runs all three against [0, 1, 5, 10, 100, 1000] and
  cross-checks the loop/formula versions against much larger values
  (1_000_000, 12345678); the recursive version is intentionally skipped
  for those, since it would overflow the call stack.

Run: node src/problem1/sum_to_n.js

Problem 2 — Fancy Form (currency swap)

src/problem2/ — Vite + React + TypeScript.

• Fetches live prices from interview.switcheo.com/prices.json; the feed has
  duplicate/stale rows per currency, so only the latest positive-price entry
  per currency is kept. A bundled snapshot (src/prices.json) is the fallback
  if the live fetch fails.
• Token icons pulled from Switcheo/token-icons;
  a currency without a matching icon falls back to an initials badge.
• Output amount is derived live from the exchange rate — no separate
  "calculate" step.
• Validation: amount must be a positive number; the two sides can't be the
  same token (enforced in the token picker itself).
• ⇅ button reverses the swap direction, carrying the current output amount
  over as the new input.
• Submit is simulated (setTimeout) since there's no real backend, so the
  loading spinner on the submit button is visible before a success message.
• Form state/logic lives in hooks/useSwapForm.ts; SwapForm.tsx is purely
  presentational.

Run: cd src/problem2 && npm install && npm run dev

Problem 3 — Messy React

src/problem3/analysis.md

12 issues identified, including a couple of real bugs (an undefined
lhsPriority reference, and an inverted amount filter that keeps
zero/negative balances instead of positive ones), plus inefficiencies
(unstable getPriority, unnecessary useMemo dependency, a dead
formattedBalances variable computed but never used, key={index} on a
reorderable list, etc.). A refactored version is included with an explanation
of each change.

Assumptions

• Problem 1: n >= 0, consistent with "sum to n" having no natural meaning
  for negative n; result stays under Number.MAX_SAFE_INTEGER per the spec.
• Problem 2: no real backend exists, so submission is mocked; WBTC/ETH
  style live prices are trusted as-is from the given endpoint without
  additional slippage/fee modeling.