1. Many slides borrowed lovingly from Skiena
Divide and Conquer
Anya E Vostinar
Grinnell College
Many slides borrowed lovingly from Skiena

2. Outline Homework reqs Divide and Conquer Find the median
Implementation on HackerRank
No office hours today

3. Homework requirements
Above all, it must run without error
If names are specified in assignment, you must use them (for my testing)
Include instructions in a README for how to run everything, make my life easy
Every function must have a header comment explaining what it does
Use sensible variable and function names
Make up ONLY THIS FIRST TIME due 9/11/2017 at 10:30pm

4. What is divide and conquer
Originally: military strategy
Now: also an algorithm design technique
Divide problem into (two?) smaller problems
Solve them recursively
Meld two partial solutions into one solution
Which should take more time: Solving the (two?) subproblems or merging them?
What are some examples?

5. Find the Median
Given a set of ‘n’ unordered numbers we want to find the ”kth” smallest number (k is an integer between 1 and n)
Talk to your neighbor about how to find the median in a given list of numbers

6. Sort it! Sort the list, what is the Big-Oh?
Let’s do better using divide and conquer!

7. Divide and Conquer recipe
Split the problem into smaller problems
How?
Solve those smaller problems
Combine the solved smaller problems
Big-Oh of worst case? Average case?

8. Divide and Conquer recipe
Split the problem into smaller problems
Randomly!
Solve those smaller problems
Put everything onto proper side of pivot, recurse on correct side if needed
Combine the solved smaller problems
Don’t need to!
Big-Oh: worst case n^2, average case n, so generally used

9. Implement it!
Go to median/problem to implement it yourself