<p>Given an array of integers <code>arr</code>, sort the array by performing a series of <strong>pancake flips</strong>.</p>
<p>In one pancake flip we do the following steps:</p>
<ul>
<li>Choose an integer <code>k</code> where <code>1 <= k <= arr.length</code>.</li>
<li>Reverse the sub-array <code>arr[0...k-1]</code> (<strong>0-indexed</strong>).</li>
</ul>
<p>For example, if <code>arr = [3,2,1,4]</code> and we performed a pancake flip choosing <code>k = 3</code>, we reverse the sub-array <code>[3,2,1]</code>, so <code>arr = [<u>1</u>,<u>2</u>,<u>3</u>,4]</code> after the pancake flip at <code>k = 3</code>.</p>
<p>Return <em>an array of the </em><code>k</code><em>-values corresponding to a sequence of pancake flips that sort </em><code>arr</code>. Any valid answer that sorts the array within <code>10 * arr.length</code> flips will be judged as correct.</p>
<li>All integers in <code>arr</code> are unique (i.e. <code>arr</code> is a permutation of the integers from <code>1</code> to <code>arr.length</code>).</li>
