title: How to sort in-place using the merge sort algorithm?
body: <p>I know the question is not too specific. </p>
  
  <p>All I want is someone to tell me how to convert a normal merge sort into an in-place merge sort (or a merge sort with constant extra space overhead). </p>
  
  <p>All I can find (on the net) is pages saying "it is too complex" or "out of scope of this text". </p>
  
  <blockquote>
    <p>The only known ways to merge in-place (without any extra space) are too complex to be reduced to practical program.  (taken <a href="http://www.personal.kent.edu/~rmuhamma/Algorithms/MyAlgorithms/Sorting/mergeSort.htm" rel="noreferrer">from here</a>)</p>
  </blockquote>
  
  <p>Even if it is too complex, <strong>what is the basic concept of how to make the merge sort in-place?</strong></p>