title: 2D matrix, find all the numbers on a straight path between two numbers
body: <p>If given a square 2D matrix of arbitrary size, how to find all the numbers on a path in between two selected numbers. e.g:</p>
  
  <pre><code>    |  1 |  2 |  3 |  4 |
      |----+----+----+----|
      |  5 |  6 |  7 |  8 |
      |----+----+----+----|
      |  9 | 10 | 11 | 12 |
      |----+----+----+----|
      | 13 | 14 | 15 | 16 |
  </code></pre>
  
  <ul>
  <li>for 6,16 = {11}</li>
  <li>for 3,15 = {7,11}</li>
  <li>for 9,2 = {}</li>
  <li>for 4,13 = {7,10}</li>
  <li>for 8,10 = {}</li>
  <li>for 12,9 = {11,10}</li>
  <li>etc.</li>
  </ul>