find-all-duplicates-in-an-array.md

<p>Given an array of integers, 1 &le; a[i] &le; <i>n</i> (<i>n</i> = size of array), some elements appear <b>twice</b> and others appear <b>once</b>.</p>

<p>Find all the elements that appear <b>twice</b> in this array.</p>

<p>Could you do it without extra space and in O(<i>n</i>) runtime?</p>
</p>
<p><b>Example:</b><br/>
<pre>
<b>Input:</b>
[4,3,2,7,8,2,3,1]

<b>Output:</b>
[2,3]
</pre>