---
id: 5a23c84252665b21eecc802a
title: Stream Merge
challengeType: 5
forumTopicId: 302326
---

## Description

<section id='description'>

Write a function that takes multiple sorted arrays of items, and returns one array of sorted items.

</section>

## Instructions

<section id='instructions'>

</section>

## Tests

<section id='tests'>

```yml
tests:
  - text: <code>mergeLists</code> should be a function.
    testString: assert(typeof mergeLists == 'function');
  - text: <code>mergeLists([[1, 3, 5, 9, 10], [2, 4, 6, 7, 8]])</code> should return an array.
    testString: assert(Array.isArray(mergeLists([[1, 3, 5, 9, 10], [2, 4, 6, 7, 8]])));
  - text: <code>mergeLists([[1, 3, 5, 9, 10], [2, 4, 6, 7, 8]])</code> should return <code>[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]</code>.
    testString: assert.deepEqual(mergeLists([[1, 3, 5, 9, 10], [2, 4, 6, 7, 8]]), [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
  - text: <code>mergeLists([[1, 4, 7, 10], [2, 5, 8, 11], [3, 6, 9, 12]])</code> should return <code>[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]</code>.
    testString: assert.deepEqual(mergeLists([[1, 4, 7, 10], [2, 5, 8, 11], [3, 6, 9, 12]]), [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]);
  - text: <code>mergeLists([[1, 3, 9, 14, 15, 17, 28], [7, 8, 14, 14, 23, 26, 28, 29, 30], [9, 23, 25, 29]])</code> should return <code>[1, 3, 7, 8, 9, 9, 14, 14, 14, 15, 17, 23, 23, 25, 26, 28, 28, 29, 29, 30]</code>.
    testString: assert.deepEqual(mergeLists([[1, 3, 9, 14, 15, 17, 28], [7, 8, 14, 14, 23, 26, 28, 29, 30], [9, 23, 25, 29]]), [1, 3, 7, 8, 9, 9, 14, 14, 14, 15, 17, 23, 23, 25, 26, 28, 28, 29, 29, 30]);
  - text: <code>mergeLists([[3, 14, 15], [2, 17, 18], [], [2, 3, 5, 7]])</code> should return <code>[2, 2, 3, 3, 5, 7, 14, 15, 17, 18]</code>.
    testString: assert.deepEqual(mergeLists([[3, 14, 15], [2, 17, 18], [], [2, 3, 5, 7]]), [2, 2, 3, 3, 5, 7, 14, 15, 17, 18]);
  - text: <code>mergeLists([[1, 19, 1999], [17, 33, 2999, 3000], [8, 500, 3999]])</code> should return <code>[1, 8, 17, 19, 33, 500, 1999, 2999, 3000, 3999]</code>.
    testString: assert.deepEqual(mergeLists([[1, 19, 1999], [17, 33, 2999, 3000], [8, 500, 3999]]), [1, 8, 17, 19, 33, 500, 1999, 2999, 3000, 3999]);
```

</section>

## Challenge Seed

<section id='challengeSeed'>

<div id='js-seed'>

```js
function mergeLists(lists) {
  // Good luck!
}
```

</div>
</section>

## Solution

<section id='solution'>

```js
function mergeLists(lists) {
  function merge(l1, l2) {
    var result = [],
      i = 0,
      j = 0;
    while (l1.length && l2.length) {
      if (l1[i] <= l2[j]) {
        result.push(l1.shift());
      } else {
        result.push(l2.shift());
      }
    }

    result.push.apply(result, l1);
    result.push.apply(result, l2);
    return result;
  }

  var result = lists[0];
  for (var i = 1; i < lists.length; i++) {
    result = merge(result, lists[i]);
  }

  return result;
}
```

</section>