[FE] Code challenge - Nguyen Ha Duy

[duyngha]

Summary

Solutions for code challenge problems 1–3.

• Problem 1 — three sum_to_n(n) implementations
• Problem 2 — currency swap form (Vite + React + TS + Tailwind)
• Problem 3 — code review & refactor of the provided WalletPage

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Problem 1 — sum_to_n

Three distinct implementations of sum_to_n(n) in src/problem1/index.js:

Function	Approach	Time	Space
sum_to_n_a	Iterative for loop	O(n)	O(1)
sum_to_n_b	Closed-form n*(n+1)/2	O(1)	O(1)
sum_to_n_c	Recursion	O(n)	O(n)

Run:

node src/problem1/index.js

See src/problem1/README.md for tradeoffs.

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Problem 2 — Currency Swap Form

A polished, Swap UI in src/problem2/.

Stack: Vite + React 19 + TypeScript + Tailwind CSS v4, with shadcn/ui.

Features:

• Live token list and prices from interview.switcheo.com/prices.json (deduped; tokens without a positive price are omitted)
• Searchable token combobox with keyboard navigation
• Auto-computed exchange rate and USD subtotal
• Swap-direction toggle (auto-swaps the pair if the user picks the same token on the opposite side)
• Inline validation (empty / non-numeric / zero / same-token)
• Skeleton while loading; retry button on fetch failure
• Simulated submit with a 1.5s loading state and success message
• Light/dark theme (toggle with d), responsive

Run:

cd src/problem2
npm install
npm run dev

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Problem 3 — WalletPage Refactor

Code review and refactor of the provided snippet, in src/problem3/.

README.md documents 14 issues grouped by severity:

• Bugs: undefined lhsPriority, inverted filter (<= 0), incomplete sort comparator, unused formattedBalances, missing blockchain field on WalletBalance
• Performance: prices in wrong useMemo deps, getPriority redeclared per render, redundant iteration passes
• Type safety: any on getPriority, lying FormattedWalletBalance cast, array index as key
• Maintainability: redundant React.FC + props: Props, empty interface, unused children

WalletPage.tsx is a clean reference refactor: priority lookup map hoisted out of the component, two memos split by dependency (filter/sort uses only balances; format/USD uses prices), stable keys on currency, no any, no dead code.

This folder is intentionally not a runnable app — imports reference assumed host-app symbols (@mui/material, hooks, WalletRow). See the README for full details.