freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/rosetta-code/longest-common-subsequence.md

id	title	challengeType	forumTopicId
5e6dd1278e6ca105cde40ea9	Longest common subsequence	5	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

tests:
  - text: <code>lcs</code> should be a function.
    testString: assert(typeof lcs == 'function');
  - text: <code>lcs("thisisatest", "testing123testing")</code> should return a string.
    testString: assert(typeof lcs("thisisatest", "testing123testing") == 'string');
  - text: <code>lcs("thisisatest", "testing123testing")</code> should return <code>"tsitest"</code>.
    testString: assert.equal(lcs("thisisatest", "testing123testing"), "tsitest");
  - text: <code>lcs("ABCDGH", "AEDFHR")</code> should return <code>"ADH"</code>.
    testString: assert.equal(lcs("ABCDGH", "AEDFHR"), "ADH");
  - text: <code>lcs("AGGTAB", "GXTXAYB")</code> should return <code>"GTAB"</code>.
    testString: assert.equal(lcs("AGGTAB", "GXTXAYB"), "GTAB");
  - text: <code>lcs("BDACDB", "BDCB")</code> should return <code>"BDCB"</code>.
    testString: assert.equal(lcs("BDACDB", "BDCB"), "BDCB");
  - text: <code>lcs("ABAZDC", "BACBAD")</code> should return <code>"ABAD"</code>.
    testString: assert.equal(lcs("ABAZDC", "BACBAD"), "ABAD");

Challenge Seed

function lcs(a, b) {

}

Solution

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;
  }
}