---
id: 5e6dd1278e6ca105cde40ea9
title: Longest common subsequence
challengeType: 5
forumTopicId: 385271
---
## Description
The longest common subsequence (or LCS) 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":
1234
1224533324
For a string example, consider the sequences "thisisatest" and "testing123testing". An LCS would be "tsitest":
thisisatest
testing123testing.
Your code only needs to deal with strings.
For more information on this problem please see Wikipedia.
## Instructions
Write a case-sensitive function that returns the LCS of two strings. You don't need to show multiple LCS's.
## Tests
``` yml
tests:
- text: lcs should be a function.
testString: assert(typeof lcs == 'function');
- text: lcs("thisisatest", "testing123testing") should return a string.
testString: assert(typeof lcs("thisisatest", "testing123testing") == 'string');
- text: lcs("thisisatest", "testing123testing") should return "tsitest".
testString: assert.equal(lcs("thisisatest", "testing123testing"), "tsitest");
- text: lcs("ABCDGH", "AEDFHR") should return "ADH".
testString: assert.equal(lcs("ABCDGH", "AEDFHR"), "ADH");
- text: lcs("AGGTAB", "GXTXAYB") should return "GTAB".
testString: assert.equal(lcs("AGGTAB", "GXTXAYB"), "GTAB");
- text: lcs("BDACDB", "BDCB") should return "BDCB".
testString: assert.equal(lcs("BDACDB", "BDCB"), "BDCB");
- text: lcs("ABAZDC", "BACBAD") should return "ABAD".
testString: assert.equal(lcs("ABAZDC", "BACBAD"), "ABAD");
```
## Challenge Seed
```js
function lcs(a, b) {
}
```
## Solution
```js
function lcs(a, b) {
var aSub = a.substr(0, a.length - 1);
var bSub = b.substr(0, b.length - 1);
if (a.length === 0 || b.length === 0) {
return '';
} else if (a.charAt(a.length - 1) === b.charAt(b.length - 1)) {
return lcs(aSub, bSub) + a.charAt(a.length - 1);
} else {
var x = lcs(a, bSub);
var y = lcs(aSub, b);
return (x.length > y.length) ? x : y;
}
}
```