Overview
Math Problem Out of Nowhere 1. We showed that $N! \in \Omega((N/2)^(N/2))$.
Math Problem Out of Nowhere 2. We showed that $log(N!) \in \Omega(N log N)$, and that $N log N \in \Omega(log(N!))$. Therefore $log(N!) \in \Theta(N log N)$.
Seeking a Sorting Lower Bound. We’ve found a number of sorts that complete execution in Θ(N log N) time. This raises the obvious question: Could we do better? Let TUCS (which stands for “The Ultimate Comparison Sort”) be the best possible algorithm that compares items and puts them in order. We know that TUCS’s worst case runtime is O(N log N) because we already know several algorithm whose worst case runtime is Θ(N log N), and TUCS’s worst case runtime is Ω(N) because we have to at least look at every item. Without further discussion, this analysis so far suggest that might be able to do better than Θ(N log N) worst case time.
Establishing a Sorting Lower Bound. As a fanciful exercise, we played a game called puppycatdog, in which we have to identify which of three boxes contains a puppy, cat, or dog. Since there are 3! = 6 permutations, we need at least ceil(lg(6)) = 3 questions to resolve the answer. In other words, if playing a game of 20 questions with 6 possible answers, we have to ask at least 3 questions to be sure we have the right answer. Since sorting through comparisons is one way to solve puppycatdog, then any lower bound on the number of comparisons for puppycatdog also applies to sorting. Given N items, there are N! permutations, meaning we need lg(N!) questions to win the game of puppycatdog, and by extension, we need at least lg(N!) to sort N items with yes/no questions. Since $log(N!) = \Theta(N log N)$, we can say that the hypothetical best sorting algorithm that uses yes/no questions requires Ω(N log N) yes/no questions. Thus, there is no comparison based algorithm that has a worst case that is a better order of growth than Θ(N log N) compares.
Recommended Problems
C level
 Problem 3 from my Fall 2014 midterm.
B level

Make sure that you understand exactly why I chose Ω, Θ, and O in “Seeking a Sorting Lower Bound” and “Establishing a Sorting Lower Bound” above.

My Fall 2013 midterm, problem 7, particularly part b.

Answer the questions from the Sounds of Sorting lecture slide.
A level

Find the optimal decision tree for playing puppy, cat, dog, walrus.