title: How to find permutation of k in a given length?
body: <p>How can I find the permutations of k in a given length?</p>
  
  <p>For example:</p>
  
  <p>The word <code>cat</code> has 3 letters: How can I find all the permutations of 2 in the word <code>cat</code>.
  Result should be: <code>ac</code>, <code>at</code>, <code>ca</code>, <code>ac</code>, etc...</p>
  
  <hr>
  
  <p>This is not a homework problem.
  Any language could be used but more preferable: C/C++ or C#. 
  I know how to create the recursion for size LENGTH but not for a custom size.</p>