[FE] - VO MINH TOAN (toanvo7120@gmail.com)

src/problem1/sum-to-n.js

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+/**
+ * Problem 1: Three Ways to Sum to N
+ *
+ * Input:  n — a positive integer
+ * Output: 1 + 2 + 3 + ... + n
+ *
+ * Assumption: input always produces result < Number.MAX_SAFE_INTEGER
+ */
+
+// =============================================================================
+// Implementation A: Gauss's Formula (Mathematical)
+// =============================================================================
+/**
+ * Uses the closed-form arithmetic series formula: n * (n + 1) / 2
+ *
+ * Explanation:
+ *   Pair the sequence from both ends: (1+n) + (2+n-1) + ...
+ *   There are n/2 such pairs, each summing to (n+1), giving n*(n+1)/2.
+ *   No loops needed — a single arithmetic expression does the job.
+ *
+ * Time Complexity:  O(1)
+ * Space Complexity: O(1)
+ */
+var sum_to_n_a = function (n) {
+  return (n * (n + 1)) / 2;
+};
+
+// =============================================================================
+// Implementation B: Iterative Loop
+// =============================================================================
+/**
+ * Accumulates the sum by looping from 1 up to n.
+ *
+ * Explanation:
+ *   The most straightforward approach — iterate and add each integer.
+ *   Easy to read, trace, and debug; no recursion overhead.
+ *
+ * Time Complexity:  O(n)
+ * Space Complexity: O(1)
+ */
+var sum_to_n_b = function (n) {
+  let sum = 0;
+  for (let i = 1; i <= n; i++) {
+    sum += i;
+  }
+  return sum;
+};
+
+// =============================================================================
+// Implementation C: Recursive
+// =============================================================================
+/**
+ * Recursively defines the sum via: sum(n) = n + sum(n - 1), base: sum(0) = 0
+ *
+ * Explanation:
+ *   Naturally mirrors the mathematical definition of the summation.
+ *   Each call reduces n by 1 until the base case is reached.
+ *   Note: not ideal for very large n due to call stack limits.
+ *
+ * Time Complexity:  O(n)
+ * Space Complexity: O(n)
+ */
+var sum_to_n_c = function (n) {
+  if (n === 0) return 0;
+  return n + sum_to_n_c(n - 1);
+};
+
+// =============================================================================
+// Test Suite
+// =============================================================================
+
+const implementations = [
+  { name: "sum_to_n_a (Gauss Formula) ", fn: sum_to_n_a },
+  { name: "sum_to_n_b (Iterative Loop)", fn: sum_to_n_b },
+  { name: "sum_to_n_c (Recursive)     ", fn: sum_to_n_c },
+];
+
+const testCases = [
+  { input: 1, expected: 1, label: "n=1   → 1" },
+  { input: 5, expected: 15, label: "n=5   → 1+2+3+4+5" },
+  { input: 10, expected: 55, label: "n=10  → 1+...+10" },
+  { input: 100, expected: 5050, label: "n=100 → Gauss classic" },
+  { input: 0, expected: 0, label: "n=0   → empty sum" },
+];
+
+let allPassed = true;
+
+console.log("=".repeat(58));
+console.log("  Running Tests for sum_to_n implementations");
+console.log("=".repeat(58));
+
+for (const { name, fn } of implementations) {
+  console.log(`\n>> ${name}`);
+  console.log("-".repeat(58));
+
+  for (const { input, expected, label } of testCases) {
+    const result = fn(input);
+    const passed = result === expected;
+    if (!passed) allPassed = false;
+
+    const status = passed ? "PASS" : "FAIL";
+    console.log(
+      `  [${status}]  ${label.padEnd(28)} got: ${result}, expected: ${expected}`
+    );
+  }
+}
+
+console.log("\n" + "=".repeat(58));
+console.log(allPassed ? "  ALL TESTS PASSED" : "  SOME TESTS FAILED");
+console.log("=".repeat(58));

src/problem2/fancy-form/.gitignore

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+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+lerna-debug.log*
+
+node_modules
+dist
+dist-ssr
+*.local
+
+# Editor directories and files
+.vscode/*
+!.vscode/extensions.json
+.idea
+.DS_Store
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?

src/problem2/fancy-form/README.md

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+# React + TypeScript + Vite
+
+This template provides a minimal setup to get React working in Vite with HMR and some ESLint rules.
+
+Currently, two official plugins are available:
+
+- [@vitejs/plugin-react](https://github.com/vitejs/vite-plugin-react/blob/main/packages/plugin-react) uses [Oxc](https://oxc.rs)
+- [@vitejs/plugin-react-swc](https://github.com/vitejs/vite-plugin-react/blob/main/packages/plugin-react-swc) uses [SWC](https://swc.rs/)
+
+## React Compiler
+
+The React Compiler is not enabled on this template because of its impact on dev & build performances. To add it, see [this documentation](https://react.dev/learn/react-compiler/installation).
+
+## Expanding the ESLint configuration
+
+If you are developing a production application, we recommend updating the configuration to enable type-aware lint rules:
+
+```js
+export default defineConfig([
+  globalIgnores(['dist']),
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [
+      // Other configs...
+
+      // Remove tseslint.configs.recommended and replace with this
+      tseslint.configs.recommendedTypeChecked,
+      // Alternatively, use this for stricter rules
+      tseslint.configs.strictTypeChecked,
+      // Optionally, add this for stylistic rules
+      tseslint.configs.stylisticTypeChecked,
+
+      // Other configs...
+    ],
+    languageOptions: {
+      parserOptions: {
+        project: ['./tsconfig.node.json', './tsconfig.app.json'],
+        tsconfigRootDir: import.meta.dirname,
+      },
+      // other options...
+    },
+  },
+])
+```
+
+You can also install [eslint-plugin-react-x](https://github.com/Rel1cx/eslint-react/tree/main/packages/plugins/eslint-plugin-react-x) and [eslint-plugin-react-dom](https://github.com/Rel1cx/eslint-react/tree/main/packages/plugins/eslint-plugin-react-dom) for React-specific lint rules:
+
+```js
+// eslint.config.js
+import reactX from 'eslint-plugin-react-x'
+import reactDom from 'eslint-plugin-react-dom'
+
+export default defineConfig([
+  globalIgnores(['dist']),
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [
+      // Other configs...
+      // Enable lint rules for React
+      reactX.configs['recommended-typescript'],
+      // Enable lint rules for React DOM
+      reactDom.configs.recommended,
+    ],
+    languageOptions: {
+      parserOptions: {
+        project: ['./tsconfig.node.json', './tsconfig.app.json'],
+        tsconfigRootDir: import.meta.dirname,
+      },
+      // other options...
+    },
+  },
+])
+```

src/problem2/fancy-form/eslint.config.js

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+import js from '@eslint/js'
+import globals from 'globals'
+import reactHooks from 'eslint-plugin-react-hooks'
+import reactRefresh from 'eslint-plugin-react-refresh'
+import tseslint from 'typescript-eslint'
+import { defineConfig, globalIgnores } from 'eslint/config'
+
+export default defineConfig([
+  globalIgnores(['dist']),
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [
+      js.configs.recommended,
+      tseslint.configs.recommended,
+      reactHooks.configs.flat.recommended,
+      reactRefresh.configs.vite,
+    ],
+    languageOptions: {
+      globals: globals.browser,
+    },
+  },
+])

src/problem2/fancy-form/index.html

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+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <link rel="icon" type="image/svg+xml" href="/vite.svg" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta name="description" content="NinetyNine DEX — Swap any token instantly at the best on-chain rates. Non-custodial, trustless, lightning-fast." />
+    <meta name="theme-color" content="#0b0d14" />
+    <title>NinetyNine DEX — Token Swap</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/main.tsx"></script>
+  </body>
+</html>