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Friday Algorithms: JavaScript Merge Sort

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Friday Algorithms: JavaScript Merge Sort

· Web Dev Zone ·
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Merge Sort


This week I’m going to cover one very popular sorting algorithm – the merge sort. It’s very intuitive and simple as it’s described in Wikipedia:

  • If the list is of length 0 or 1, then it is already sorted. Otherwise:
  • Divide the unsorted list into two sublists of about half the size.
  • Sort each sublist recursively by re-applying merge sort.
  • Merge the two sublists back into one sorted list.


Here’s the Source

(JavaScript)

var a = [34, 203, 3, 746, 200, 984, 198, 764, 9];

function mergeSort(arr)
{
if (arr.length < 2)
return arr;

var middle = parseInt(arr.length / 2);
var left = arr.slice(0, middle);
var right = arr.slice(middle, arr.length);

return merge(mergeSort(left), mergeSort(right));
}

function merge(left, right)
{
var result = [];

while (left.length && right.length) {
if (left[0] <= right[0]) {
result.push(left.shift());
} else {
result.push(right.shift());
}
}

while (left.length)
result.push(left.shift());

while (right.length)
result.push(right.shift());

return result;
}

console.log(mergeSort(a));

It’s interesting to see what happens in the Firebug’s console:

[34, 203, 3, 746] [200, 984, 198, 764, 9]

[34, 203] [3, 746]

[34] [203]

[3] [746]

[200, 984] [198, 764, 9]

[200] [984]

[198] [764, 9]

[764] [9]

[3, 9, 34, 198, 200, 203, 746, 764, 984]

Actually the tricky part in this algorithm is the merge function – it does all the work.

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