Reading

These notes and closely review Unit 2 Section 2.

Quiz questions

Review quiz questions

Both reviews were quite important!

Review: Applying Divide & Conquer to sorting

Next we decided to try applying Divide and Conquer to the problem of sorting. This is where we got to:
Algorithm: sort(A,i,j)

if i == j
  return

m = (i + j)/2          ← divide

r1 = sort(A,i,m)       ← conquer
r2 = sort(A,m+1,j)

?????                  ← combine
      
So the question is, how do we efficiently combine these two sorted subarrays to form the sorted version of the original array? The answer: I wish I had an algorithm "merge" that would do that for me!

Merge

Ask and you shall receive, here is an algorithm for merging two consecutive sorted subranges.
Algorithm: merge(A,i,m,j)
Inputs : array A, and indices i ≤ m ≤ j such that
         A[i..m] and A[(m+1)..j] are idividually in sorted order
Outputs: array A is modified so that  A[i..j] contains its original
         elements in sorted order

A1 = new array copy of A[i..m]
A2 = new array copy of A[(m+1)..j]

k = i, i1 = 0, j1 = m - i, i2 = 0, j2 = j - (m+1)

while i1 ≤ j1 and i2 ≤ j2 do            ← keep adding the smaller of the initial elements of A1 & A2
  if A2[i2] < A2[i1]
    A[k] = A2[i2], i2 = i2 + 1, k = k + 1
  else
    A[k] = A1[i1], i1 = i1 + 1, k = k + 1

while i1 ≤ j1 do                        ← add any remaining elements of A1
  A[k] = A1[i1], i1 = i1 + 1, k = k + 1

while i2 ≤ j2 do                        ← add any remaining elements of A2
  A[k] = A2[i1], i1 = i1 + 1, k = k + 1

free A1 and A2
      
This version of merge should be fairly easy to understand, so I won't worry about proving correctness. What about its running time? Best and worst case running times are both $\Theta(n)$, which we derived in class (and is done in the unit notes).

Mergesort completed ... and a start on analysis

So here's mergesort, a divide and conquer sorting algorithm.
Algorithm: msort(A,i,j)

if i == j
  return

m = (i + j)/2           ← divide

r1 = msort(A,i,m)       ← conquer
r2 = msort(A,m+1,j)

merge(A,i,m,j)          ← combine
      
In class we made a start on analyzing mergesort. If we use $T(n)$ to denote mergesort's worst-case running time as a function of the input array size, we end up with the following:

$ T(n) \leq c n + T\left( \lceil n/2 \rceil \right) + T\left( \lfloor n/2 \rfloor \right) $

where the floor and ceiling are required because with an odd-sized range, we cannot divide evenly in half. This is a bit difficult to analyze, so we said instead that we would try to examine a simplified version. We expaned the simpler version out and looked for patterns, and arrived at the hypothesis that $T(n) \in O(n \lg n)$. Next class: prove this hypothess!