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<a href="http://rosettacode.org/wiki/100_doors">http://rosettacode.org/wiki/100_doors</a>
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<ul>
<li>There are 100 doors in a row that are all initially closed.</li>
<li>You make 100 passes by the doors.</li>
<li>The first time through, visit every door and toggle the door (if the door is closed, open it; if it is open, close it).</li>
<li>The second time, only visit every 2nd door (door #2, #4, #6, ...), and toggle it.</li>
<li>The third time, visit every 3rd door (door #3, #6, #9, ...), etc, until you only visit the 100th door.</li>
</ul>
What state are the doors in after the last pass? Which are open, which are closed?

