title: get the number of overlapping intervals, given two lists of intervals
body: <p>I recently came across an interesting problem:</p>
  
  <p>Given two lists of intervals, find the total number of overlapping intervals from the two lists. </p>
  
  <pre><code>Example
  
  L1: ([1,2][2,3][4,5][6,7])
  L2: ([1,5][2,3][4,7][5,7])
  
  [1,5] overlaps [1,2] [2,3] [4,5]
  [2,3] overlaps [1,2] [2,3]
  [4,7] overlaps [4,5] [6,7]
  [5,7] overlaps [4,5] [6,7]
  
  total = 3+2+2+2 = 9
  </code></pre>
  
  <p>Obviously the brute force approach works, but it's too slow (I need something better than O(n^2)).</p>
  
  <p>I also fond a similar problem <a href="http://www.geeksforgeeks.org/minimum-number-platforms-required-railwaybus-station/" rel="nofollow">here</a>. But it's not exactly the same...</p>
  
  <p>Any help is appreciated</p>