[FE] - PHUONG BUI THANH (Michael - btphuong2017@gmail.com)

[btphuong2017]

Solutions — 99Tech Code Challenge #1

Problem 1: Three ways to sum to n

Location: src/problem1/index.ts

Three distinct implementations of sum_to_n(n), each returning 1 + 2 + ... + n. For n ≤ 0 all three consistently return 0 (empty sum).

	Approach	Complexity
A	Iterative for loop	O(n) time, O(1) space
B	Gauss closed form n*(n+1)/2	O(1) time, O(1) space
C	Array.from + reduce	O(n) time, O(n) space

Implementation B is the practical choice for any real usage. Implementation C demonstrates a functional style that composes naturally with further sequence transforms.

---

Problem 2: Fancy Form

Location: src/problem2/ — run with npm install && npm run dev

A currency swap form built with Vite + React + TypeScript (bonus stack).

Key features:

• Fetches live token prices from the Switcheo API on load; deduplicates multiple records per currency by keeping the most recent date entry. Tokens without a price are omitted.
• Token icons loaded from the Switcheo CDN; missing icons are hidden gracefully.
• Real-time receive-amount computation: receiveAmount = sendAmount × (fromPrice / toPrice).
• Swap-direction flip button; inline validation (positive amount, both tokens selected, tokens must differ).
• Confirm Swap simulates a backend call with a 1.5 s loading spinner → success banner → form reset.

Unit tests for the pure lib/ logic (exchange-rate math, price deduplication/filtering) are in src/lib/*.test.ts and run with npm test.

---

Problem 3: Messy React

Location: src/problem3/

README.md lists 13 issues found in the original WalletPage component, ordered from most to least severe:

1. lhsPriority is not defined → runtime ReferenceError (should be balancePriority)
2. Filter logic is inverted — keeps amount <= 0 instead of > 0
3. WalletBalance interface missing the blockchain field used in the code
4. getPriority re-created on every render (should be hoisted to module scope)
5. blockchain: any parameter discards type safety
6. prices in useMemo deps but never used by the filter/sort
7. Sort comparator returns undefined for equal priorities
8. rows maps over sortedBalances but casts to FormattedWalletBalance → balance.formatted is always undefined
9. formattedBalances not memoised — runs on every render
10. Array index used as React key
11. prices[balance.currency] unguarded → potential NaN
12. toFixed() called without a precision argument
13. children destructured but never used

WalletPage.tsx is the refactored version: typed Blockchain union, getPriority hoisted with a never exhaustiveness check, a single useMemo that filters, sorts, and formats in one pass, stable composite keys, and guarded usdValue.