title: what is a good data structure to store a permutation and how many times that permutation occurs?
body: <p>my permutations are unique pairs of numbers. I need to go through a list and count the number of times a given permutation occurs. So i need to record it in a data structure.</p>
  
  <pre><code>(1,1) 0
  (1,2) 5
  (1,3) 3
  (2,2) 5
  (2,3) 2
  (3,3) 1
  </code></pre>
  
  <p>Ultimately, I need to be able to sort this container in the decreasing order so I can get the permutations which occurs the maximum number of times.
  Thanks a lot!!</p>