22afc2a0ca
Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com> Co-authored-by: Kristofer Koishigawa <scissorsneedfoodtoo@gmail.com> Co-authored-by: Beau Carnes <beaucarnes@gmail.com>
82 lines
1.8 KiB
Markdown
82 lines
1.8 KiB
Markdown
---
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id: 5900f3801000cf542c50fe93
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challengeType: 5
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isHidden: false
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title: 'Problem 20: Factorial digit sum'
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forumTopicId: 301839
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---
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## Description
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<section id='description'>
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<var>n</var>! means <var>n</var> × (<var>n</var> − 1) × ... × 3 × 2 × 1
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For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,<br>and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
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Find the sum of the digits `n`!
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</section>
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## Instructions
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<section id='instructions'>
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: <code>sumFactorialDigits(10)</code> should return a number.
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testString: assert(typeof sumFactorialDigits(10) === 'number');
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- text: <code>sumFactorialDigits(10)</code> should return 27.
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testString: assert.strictEqual(sumFactorialDigits(10), 27);
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- text: <code>sumFactorialDigits(25)</code> should return 72.
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testString: assert.strictEqual(sumFactorialDigits(25), 72);
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- text: <code>sumFactorialDigits(50)</code> should return 216.
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testString: assert.strictEqual(sumFactorialDigits(50), 216);
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- text: <code>sumFactorialDigits(75)</code> should return 432.
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testString: assert.strictEqual(sumFactorialDigits(75), 432);
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- text: <code>sumFactorialDigits(100)</code> should return 648.
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testString: assert.strictEqual(sumFactorialDigits(100), 648);
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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```js
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function sumFactorialDigits(n) {
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// Good luck!
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return n;
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}
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sumFactorialDigits(100);
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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let factorial = (n) => n <= 1 ? BigInt(n) : BigInt(n) * BigInt(factorial(--n));
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let sumDigits = n => n.toString().split('').map(x => parseInt(x)).reduce((a,b) => a + b);
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function sumFactorialDigits(n) {
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return sumDigits(factorial(n));
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}
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```
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</section>
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