84 lines
2.8 KiB
Markdown
84 lines
2.8 KiB
Markdown
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---
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id: 5e6dd1278e6ca105cde40ea9
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title: Longest common subsequence
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challengeType: 5
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isHidden: false
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---
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## Description
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<section id='description'>
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The <b>longest common subsequence</b> (or <a href="http://en.wikipedia.org/wiki/Longest_common_subsequence_problem" target="_blank"><b>LCS</b></a>) of groups A and B is the longest group of elements from A and B that are common between the two groups and in the same order in each group. For example, the sequences "1234" and "1224533324" have an LCS of "1234":
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<b><u>1234</u></b>
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<b><u>12</u></b>245<b><u>3</u></b>332<b><u>4</u></b>
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For a string example, consider the sequences "thisisatest" and "testing123testing". An LCS would be "tsitest":
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<b><u>t</u></b>hi<b><u>si</u></b>sa<b><u>test</u></b>
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<b><u>t</u></b>e<b><u>s</u></b>t<b><u>i</u></b>ng123<b><u>test</u></b>ing.
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Your code only needs to deal with strings.
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For more information on this problem please see <a href="https://en.wikipedia.org/wiki/Longest_common_subsequence_problem" target="_black">Wikipedia</a>.
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</section>
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## Instructions
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<section id='instructions'>
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Write a case-sensitive function that returns the LCS of two strings. You don't need to show multiple LCS's.
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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>lcs</code> should be a function.
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testString: assert(typeof lcs == 'function');
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- text: <code>lcs("thisisatest", "testing123testing")</code> should return a string.
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testString: assert(typeof lcs("thisisatest", "testing123testing") == 'string');
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- text: <code>lcs("thisisatest", "testing123testing")</code> should return <code>"tsitest"</code>.
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testString: assert.equal(lcs("thisisatest", "testing123testing"), "tsitest");
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- text: <code>lcs("ABCDGH", "AEDFHR")</code> should return <code>"ADH"</code>.
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testString: assert.equal(lcs("ABCDGH", "AEDFHR"), "ADH");
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- text: <code>lcs("AGGTAB", "GXTXAYB")</code> should return <code>"GTAB"</code>.
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testString: assert.equal(lcs("AGGTAB", "GXTXAYB"), "GTAB");
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- text: <code>lcs("BDACDB", "BDCB")</code> should return <code>"BDCB"</code>.
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testString: assert.equal(lcs("BDACDB", "BDCB"), "BDCB");
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- text: <code>lcs("ABAZDC", "BACBAD")</code> should return <code>"ABAD"</code>.
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testString: assert.equal(lcs("ABAZDC", "BACBAD"), "ABAD");
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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 lcs(a, b) {
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}
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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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function lcs(a, b) {
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var aSub = a.substr(0, a.length - 1);
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var bSub = b.substr(0, b.length - 1);
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if (a.length === 0 || b.length === 0) {
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return '';
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} else if (a.charAt(a.length - 1) === b.charAt(b.length - 1)) {
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return lcs(aSub, bSub) + a.charAt(a.length - 1);
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} else {
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var x = lcs(a, bSub);
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var y = lcs(aSub, b);
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return (x.length > y.length) ? x : y;
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}
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}
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```
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</section>
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