title: Interleave array in constant space
body: <p>Suppose we have an array
  a1, a2,... , an, b1, b2, ..., bn.</p>
  <p>The goal is to change this array to
  a1, b1, a2, b2, ..., an, bn in O(n) time and in O(1) space.
  In other words, we need a linear-time algorithm to modify the array in place, with no more than a constant amount of extra storage.</p>
  <p>How can this be done?</p>