93 lines
2.1 KiB
Markdown
93 lines
2.1 KiB
Markdown
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---
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id: 5e6dd14797f5ce267c2f19d0
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title: Look-and-say sequence
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challengeType: 5
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forumTopicId: 385277
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dashedName: look-and-say-sequence
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---
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# --description--
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The [Look and say sequence](https://en.wikipedia.org/wiki/Look and say sequence) is a recursively defined sequence of numbers.
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Sequence Definition
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<ul><li>Take a decimal number</li>
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<li><span>Look</span> at the number, visually grouping consecutive runs of the same digit.</li>
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<li><span>Say</span> the number, from left to right, group by group; as how many of that digit there are - followed by the digit grouped.</li></ul><span> This becomes the next number of the sequence.</span>
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An example:
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<ul><li>Starting with the number 1, you have <span>one</span> 1 which produces 11</li>
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<li>Starting with 11, you have <span>two</span> 1's. I.E.: 21</li>
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<li>Starting with 21, you have <span>one</span> 2, then <span>one</span> 1. I.E.: (12)(11) which becomes 1211</li>
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<li>Starting with 1211, you have <span>one</span> 1, <span>one</span> 2, then <span>two</span> 1's. I.E.: (11)(12)(21) which becomes 111221</li></ul>
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# --instructions--
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Write a function that accepts a string as a parameter, processes it, and returns the resultant string.
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# --hints--
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`lookAndSay` should be a function.
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```js
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assert(typeof lookAndSay == 'function');
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```
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`lookAndSay("1")` should return a string.
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```js
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assert(typeof lookAndSay('1') == 'string');
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```
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`lookAndSay("1")` should return `"11"`.
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```js
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assert.equal(lookAndSay('1'), '11');
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```
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`lookAndSay("11")` should return `"21"`.
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```js
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assert.equal(lookAndSay('11'), '21');
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```
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`lookAndSay("21")` should return `"1211"`.
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```js
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assert.equal(lookAndSay('21'), '1211');
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```
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`lookAndSay("1211")` should return `"111221"`.
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```js
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assert.equal(lookAndSay('1211'), '111221');
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```
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`lookAndSay("3542")` should return `"13151412"`.
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```js
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assert.equal(lookAndSay('3542'), '13151412');
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```
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# --seed--
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## --seed-contents--
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```js
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function lookAndSay(str) {
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}
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```
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# --solutions--
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```js
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function lookAndSay(str) {
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return str.replace(/(.)\1*/g, function(seq, p1) {
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return seq.length.toString() + p1;
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});
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}
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```
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