title: in-place permutation of a array follows this rule
body: <p>Suppose there is an array, we want to find everything in the odd index (index starting with 0), and move it to the end.
  Everything in the even index move it to the beginning.
  The relative order of all odd index items and all even index items are preserved.</p>
  
  <p>i.e. if the array is </p>
  
  <pre><code>a1 b1 a2 b2 ...  an bn    
  </code></pre>
  
  <p>after the operation it becomes</p>
  
  <pre><code>a1 a2 a3 ... an b1 b2 ... bn
  </code></pre>
  
  <p>Can this be done in-place and in O(n) time?</p>