title: skyline algorithm
body: <p>How do I find the vertices of the broken line that surrounds the silhouette in this image? <img src="https://imgur.com/SnSjn.gif" alt="Skyline"></p>
  
  <p>A possible input for the example above is:</p>
  
  <pre>
  WIDTH  HEIGHT  POSITION
    3       9       17
    5       9        9
   12       4        8
    3      11        3
   10       7        1
    2       3       19
  </pre>
  
  <p>So for this example the solution would be</p>
  
  <pre>
  [(1, 0), (1, 7), (3, 7), (3, 11), (6, 11), (6, 7), 
   (9, 7), (9, 9), (14, 9), (14, 4), (17, 4), (17, 9), 
   (20, 9), (20, 3), (21, 3), (21, 0)]
  </pre>