CSC151 2009F, Class 48: Merge Sort

Admin:
* No reading for Monday.  
  * I may distribute links to the project images so that we can better 
    discuss project code on Monday.
* I hope you have a wonderful Thanksgiving!

Overview:
* More efficient sorting techniques.
* Divide and conquer, revisited.
* Merge sort.
* Analyzing merge sort.

How expensive is insertion sort?
* It depends a bit on the ordering of the list
  * List-based insertion sort, we're best off having the list in
    what form?
    * Already sorted
    * Backwards sorted [The winner]
      * As we insert each element, we get to "plunk it at the front"
  * In list-based insertion sort, already sorted is the worst starting
    point
    * As we insert each element into the sorted list, we have to go
      through *everything* already in the sorted list

Divide and conquer sorting:
* Divide the list into two halves
* Sort each half
* Join the halves together

To split a list
* Find the length
  * (take lst (/ length 2))
  * (drop lst (/ length 2))q

This is recursive.  What's the base case?
  * A list of length 1 is already sorted