freeCodeCamp/curriculum/challenges/en/08-coding-interview-prep/algorithms/no-repeats-please.en.md

---
id: a7bf700cd123b9a54eef01d5
title: No Repeats Please
challengeType: 5
---

## Description
<section id='description'>
Return the number of total permutations of the provided string that don't have repeated consecutive letters. Assume that all characters in the provided string are each unique.
For example, <code>aab</code> should return 2 because it has 6 total permutations (<code>aab</code>, <code>aab</code>, <code>aba</code>, <code>aba</code>, <code>baa</code>, <code>baa</code>), but only 2 of them (<code>aba</code> and <code>aba</code>) don't have the same letter (in this case <code>a</code>) repeating.
Remember to use <a href='http://forum.freecodecamp.org/t/how-to-get-help-when-you-are-stuck/19514' target='_blank'>Read-Search-Ask</a> if you get stuck. Try to pair program. Write your own code.
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
- text: <code>permAlone("aab")</code> should return a number.
  testString: 'assert.isNumber(permAlone(''aab''), ''<code>permAlone("aab")</code> should return a number.'');'
- text: <code>permAlone("aab")</code> should return 2.
  testString: 'assert.strictEqual(permAlone(''aab''), 2, ''<code>permAlone("aab")</code> should return 2.'');'
- text: <code>permAlone("aaa")</code> should return 0.
  testString: 'assert.strictEqual(permAlone(''aaa''), 0, ''<code>permAlone("aaa")</code> should return 0.'');'
- text: <code>permAlone("aabb")</code> should return 8.
  testString: 'assert.strictEqual(permAlone(''aabb''), 8, ''<code>permAlone("aabb")</code> should return 8.'');'
- text: <code>permAlone("abcdefa")</code> should return 3600.
  testString: 'assert.strictEqual(permAlone(''abcdefa''), 3600, ''<code>permAlone("abcdefa")</code> should return 3600.'');'
- text: <code>permAlone("abfdefa")</code> should return 2640.
  testString: 'assert.strictEqual(permAlone(''abfdefa''), 2640, ''<code>permAlone("abfdefa")</code> should return 2640.'');'
- text: <code>permAlone("zzzzzzzz")</code> should return 0.
  testString: 'assert.strictEqual(permAlone(''zzzzzzzz''), 0, ''<code>permAlone("zzzzzzzz")</code> should return 0.'');'
- text: <code>permAlone("a")</code> should return 1.
  testString: 'assert.strictEqual(permAlone(''a''), 1, ''<code>permAlone("a")</code> should return 1.'');'
- text: <code>permAlone("aaab")</code> should return 0.
  testString: 'assert.strictEqual(permAlone(''aaab''), 0, ''<code>permAlone("aaab")</code> should return 0.'');'
- text: <code>permAlone("aaabb")</code> should return 12.
  testString: 'assert.strictEqual(permAlone(''aaabb''), 12, ''<code>permAlone("aaabb")</code> should return 12.'');'

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function permAlone(str) {
  return str;
}

permAlone('aab');
```

</div>



</section>

## Solution
<section id='solution'>


```js
function permAlone(str) {
  return permutor(str).filter(function(perm) {
    return !perm.match(/(.)\1/g);
  }).length;
}

function permutor(str) {
  // http://staff.roguecc.edu/JMiller/JavaScript/permute.html
  //permArr: Global array which holds the list of permutations
  //usedChars: Global utility array which holds a list of "currently-in-use" characters
  var permArr = [], usedChars = [];
  function permute(input) {
    //convert input into a char array (one element for each character)
    var i, ch, chars = input.split("");
    for (i = 0; i < chars.length; i++) {
      //get and remove character at index "i" from char array
      ch = chars.splice(i, 1);
      //add removed character to the end of used characters
      usedChars.push(ch);
      //when there are no more characters left in char array to add, add used chars to list of permutations
      if (chars.length === 0) permArr[permArr.length] = usedChars.join("");
      //send characters (minus the removed one from above) from char array to be permuted
      permute(chars.join(""));
      //add removed character back into char array in original position
      chars.splice(i, 0, ch);
      //remove the last character used off the end of used characters array
      usedChars.pop();
    }
  }
  permute(str);
  return permArr;
}

permAlone('aab');

```

</section>