vim-castle/home/.vim/vimwiki/vimwiki_html/Quicksort.html

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2015-01-14 23:41:47 +00:00
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<link rel="Stylesheet" type="text/css" href="style.css">
<title>Quicksort</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<h2 id="toc_0.1">Quicksort</h2>
<dl>
<dt>Stats</dt>
<dd>Worst Case Performance: O(n<sup><small>2</small></sup>)</dd>
<dd>Best Case Performance: O(n log n)</dd>
</dl>
<h3 id="toc_0.1.1">Algorithm</h3>
<ol>
<li>
Pick an element, called a <strong>pivot</strong>, from the list.
<li>
Make three lists:
<ul>
<li>
before: all elements smaller than pivot
<li>
pivot: all elements equal to pivot
<li>
after: all elements larger than pivot
</ul>
<li>
recurse on before-list and after-list, then return concat(before, pivot, after)
</ol>
<h3 id="toc_0.1.2">Psuedocode</h3>
<pre>
function quicksort(items) {
if(count(items) &lt;= 1) {
return a
}
middle_idx = count(items)/2;
pivot = [];
pivot[] = items[middle_idx];
before_list = after_list = [];
for(i in items) {
if(items[i] &lt; pivot) {
before_list[] = items[i];
} else if(items[i] &gt; pivot) {
after_list[] = items[i];
} else {
pivot[] = items[i];
}
}
return concatenate(quicksort(before_list), pivot, quicksort(after_list));
}
</pre>
<h3 id="toc_0.1.3">Additional Info</h3>
<p>
Wikipedia Article [<a href="http://en.wikipedia.org/wiki/Quicksort]">http://en.wikipedia.org/wiki/Quicksort]</a>
</p>
</body>
</html>