freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/rosetta-code/look-and-say-sequence.md

id, title, challengeType, forumTopicId, dashedName

id	title	challengeType	forumTopicId	dashedName
5e6dd14797f5ce267c2f19d0	Look-and-say sequence	5	385277	look-and-say-sequence

--description--

The Look and say sequence is a recursively defined sequence of numbers.

Sequence Definition

• Take a decimal number
• Look at the number, visually grouping consecutive runs of the same digit.
• Say the number, from left to right, group by group; as how many of that digit there are - followed by the digit grouped.

This becomes the next number of the sequence.

An example:

• Starting with the number 1, you have one 1 which produces 11
• Starting with 11, you have two 1's. I.E.: 21
• Starting with 21, you have one 2, then one 1. I.E.: (12)(11) which becomes 1211
• Starting with 1211, you have one 1, one 2, then two 1's. I.E.: (11)(12)(21) which becomes 111221

--instructions--

Write a function that accepts a string as a parameter, processes it, and returns the resultant string.

--hints--

lookAndSay should be a function.

assert(typeof lookAndSay == 'function');

lookAndSay("1") should return a string.

assert(typeof lookAndSay('1') == 'string');

lookAndSay("1") should return "11".

assert.equal(lookAndSay('1'), '11');

lookAndSay("11") should return "21".

assert.equal(lookAndSay('11'), '21');

lookAndSay("21") should return "1211".

assert.equal(lookAndSay('21'), '1211');

lookAndSay("1211") should return "111221".

assert.equal(lookAndSay('1211'), '111221');

lookAndSay("3542") should return "13151412".

assert.equal(lookAndSay('3542'), '13151412');

--seed--

--seed-contents--

function lookAndSay(str) {

}

--solutions--

function lookAndSay(str) {
    return str.replace(/(.)\1*/g, function(seq, p1) {
      return seq.length.toString() + p1;
    });
}