freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/rosetta-code/towers-of-hanoi.english.md

---
title: Towers of Hanoi
id: 5951ed8945deab770972ae56
challengeType: 5
---

## Description
<section id='description'>
    Task:
<p>Solve the <a href="https://en.wikipedia.org/wiki/Towers_of_Hanoi" title="wp: Towers_of_Hanoi">Towers of Hanoi</a>  problem.</p>
<p>
Your solution should accept the number of discs as the first parameters, and
three string used to identify each of the three stacks of discs, for example
<code>towerOfHanoi(4, 'A', 'B', 'C')</code>. The function should return an
array of arrays containing the list of moves, source -> destination. For
example, the array <code>[['A', 'C'], ['B', 'A']]</code> indicates that the
1st move was to move a disc from stack A to C, and the 2nd move was to move a
disc from stack B to A.
</p>
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>towerOfHanoi</code> is a function.
    testString: 'assert(typeof towerOfHanoi === "function", "<code>towerOfHanoi</code> is a function.");'
  - text: '<code>towerOfHanoi(3, ...)</code> should return 7 moves.'
    testString: 'assert(res3.length === 7, "<code>towerOfHanoi(3, ...)</code> should return 7 moves.");'
  - text: '<code>towerOfHanoi(3, "A", "B", "C")</code> should return [["A","B"],["A","C"],["B","C"],["A","B"],["C","A"],["C","B"],["A","B"]].")'
    testString: 'assert.deepEqual(towerOfHanoi(3, "A", "B", "C"), res3Moves, "<code>towerOfHanoi(3, "A", "B", "C")</code> should return [["A","B"],["A","C"],["B","C"],["A","B"],["C","A"],["C","B"],["A","B"]].");'
  - text: '<code>towerOfHanoi(5, "X", "Y", "Z")</code> 10th move should be Y -> X.'
    testString: 'assert.deepEqual(res5[9], ["Y", "X"], "<code>towerOfHanoi(5, "X", "Y", "Z")</code> 10th move should be Y -> X.");'
  - text: '<code>towerOfHanoi(7, "A", "B", "C")</code> first ten moves are [["A","B"],["A","C"],["B","C"],["A","B"],["C","A"],["C","B"],["A","B"],["A","C"],["B","C"],["B","A"]].")'
    testString: 'assert.deepEqual(towerOfHanoi(7, "A", "B", "C").slice(0, 10), res7First10Moves, "<code>towerOfHanoi(7, "A", "B", "C")</code> first ten moves are [["A","B"],["A","C"],["B","C"],["A","B"],["C","A"],["C","B"],["A","B"],["A","C"],["B","C"],["B","A"]].");'

```

</section>

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

<div id='js-seed'>

```js
function towerOfHanoi (n, a, b, c) {
  // Good luck!
  return [[]];
}
```

</div>


### After Test
<div id='js-teardown'>

```js
console.info('after the test');
```

</div>

</section>

## Solution
<section id='solution'>


```js
function towerOfHanoi(n, a, b, c) {
  const res = [];
  towerOfHanoiHelper(n, a, c, b, res);
  return res;
}

function towerOfHanoiHelper(n, a, b, c, res) {
  if (n > 0) {
    towerOfHanoiHelper(n - 1, a, c, b, res);
    res.push([a, c]);
    towerOfHanoiHelper(n - 1, b, a, c, res);
  }
}

```

</section>