QuickSort (Ch. 7) Like Merge-Sort, based on the three-step process of divide- and-conquer. Input: An array A[1…n] of comparable elements, the starting position and the ending index of A. Output: A permutation of A such that elements are in ascending order

QuickSort Divide: Rearrange the array A[p..r] into two subarrays A[p..q-1] and A[q+1..r] such that each element of A[p..q-1]  A[q] and each element of A[q+1..r] > A[q]. Conquer: Sort the two subarrays by recursive calls to QuickSort. Combine: No work is needed to combine subarrays since they are sorted in-place. To sort the subarray A[p…r] ( initially, p = 1 and r = A.length) :

QuickSort Quicksort(A, p, r) 1. if p < r 2. q = Partition(A, p, r) 3. Quicksort(A, p, q-1) 4. Quicksort(A, q+1, r) Partition(A, p, r) 1. x = A[r] // choose pivot 2. i = p for j = p to r if A[j] ≤ x 5. i = i swap A[i] and A[j] 7. swap A[i+1] and A[r] 8. return i + 1 Initial call: Quicksort(A, 1, A.length) What does Partition do? What is running time of Partition? Note that Partition always selects the last element A[r] in the subarray A[p…r] as the pivot.

Partition Partition(A, p, r) 1. x = A[r] // choose pivot 2. i = p for j = p to r if A[j] ≤ x 5. i = i swap A[i] and A[j] 7. swap A[i+1] and A[r] 8. return i + 1 In line 1, x is set to the value contained in A[r] (the pivot). i is set to index before p, j starts at p j goes through entire array segment from p to r-1; any time A[j] ≤ x, i is incremented and A[i] is exchanged with A[j] When j = r, the value at A[r] is exchanged with the value at A[i+1]. This puts the value in A[r] between the values ≤ x and those > x. So the item at position A[i+1] is in its correct sorted position. The index i+1 is returned because that position no longer needs to be considered.

Partition Partition(A, p, r) 1. x = A[r] // choose pivot 2. i = p for j = p to r if A[j] ≤ x 5. i = i swap A[i] and A[j] 7. swap A[i+1] and A[r] 8. return i + 1 Loop invariant: As Partition executes, the array is divided into 4 regions, some possibly empty, such that 1.All values in A[p…i] are ≤ pivot. 2.All values in A[i+1…j-1] are > pivot. 3.A[r] = pivot. Although not needed as part of the loop invariant, the 4 th region is A[j…r-1], whose entries have not yet been examined, so we don’t know how they compare to the pivot.

Partition Partition(A, p, r) 1. x = A[r] // choose pivot 2. i = p for j = p to r if A[j] ≤ x 5. i = i swap A[i] and A[j] 7. swap A[i+1] and A[r] 8. return i Example on an 8-element array:

Partition Partition(A, p, r) 1. x = A[r] // choose pivot 2. i = p for j = p to r if A[j] ≤ x 5. i = i swap A[i] and A[j] 7. swap A[i+1] and A[r] 8. return i + 1 The index j disappears after step 7 because it is no longer needed once the for loop is exited

Formal Correctness of Partition As the Partition procedure executes, the array is partitioned into four regions, some of which may be empty. Loop invariant: At the beginning of each iteration of the for loop (lines 3-6), for any array index k, 1.If p ≤ k ≤ i, then A[k] ≤ x. 2.If i+1 ≤ k ≤ j-1, then A[k] > x. 3.If k=r, then A[k] = x. The fourth region is A[j…r - 1], whose entries have not yet been examined. Partition(A, p, r) 1. x = A[r] // choose pivot 2. i = p for j = p to r if A[j] ≤ x 5. i = i swap A[i] and A[j] 7. swap A[i+1] and A[r] 8. return i + 1

Partition: Correctness Initialization: Before the loop starts i=0 and j=1, all the conditions of the loop invariant are satisfied because r is the pivot, and the sub-arrays A[p…i] (A[1…0]) and A[i+1…j-1] (A[1…0]) are empty. Maintenance: In each iteration of the loop, either A[j] > x or A[j] ≤ x. In the first case, j is incremented and cond. 2 holds for A[j-1] with no other changes. In the second case, i is incremented, A[i] and A[j] are swapped, and then j is incremented. Cond. 1 holds for A[i] after swap. By the IHOP, the item in A[i] was in A[i+1] during the last iteration, and was > x then, so cond. 2 holds at the end of the iteration. Termination: At termination, j = r and A has been partitioned into 3 sets: A[p…i] pivot, and A[r] = pivot. The last 2 lines of Partition move the pivot element from the highest position in the array to between the 2 sub-arrays by swapping A[i+1] and A[r].

Quicksort Running Time The running time of quicksort depends on the partitioning of subarrays. If the subarrays are balanced, then quicksort can run as fast as mergesort. If the subarrays are unbalanced, then quicksort can run as slowly as insertionsort.

Quicksort Running Time Worst case occurs when subarrays are completely unbalanced, when there are 0 elements in one subarray and n-1 elements in the other. We get the recurrence T(n) = T(n-1) + T(0) + Θ (n) = T(n-1) + Θ (n) = Θ (n 2 ) This is the same running time as the worst case of insertion sort. What input instance would cause the worst-case running time of quicksort to occur? When the array is already sorted in ascending order.

Quicksort Running Time Best case occurs when subarrays are completely balanced and each subarray has ≤ n/2 elements We get the recurrence T(n) = 2T(n/2) + Θ (n) = Θ (nlgn) by Case 2 of the master theorem. This is the same running time as merge sort.

Quicksort Average-case The average case of quicksort’s running time is much closer to the best case than it is to the worst case. Imagine that Partition always produces a 9-to-1 split. Then we get the recurrence T(n) ≤ T(9n/10) + T(n/10) + Θ (n) = O(nlgn) The recursion tree is like the one for T(n) = T(n/3) + T(2n/3) + O(n) Any split of constant proportionality will yield a recursion tree of depth Θ (lgn)

Quicksort Average-case Intuition: Some splits will be close to balanced and others close to unbalanced, so good and bad splits will be randomly distributed in recursion tree. The running time will be (asymptotically) bad only if there are many bad splits in a row. A bad split followed by a good split results in a good partitioning after one extra step.

Quicksort with Good Average Running Time How can we modify Quicksort to get good average case behavior on all inputs? Randomized-Partition(A, p, r) 1. i = Random(p, r) 2. swap A[r] and A[i] 3. return Partition(A, p, r) 2 techniques: 1.randomly permute input prior to running Quicksort. Will produce tree of possible executions, most of them finish fast. 2.choose partition randomly at each iteration instead of choosing element in highest array position. Randomized-Quicksort(A, p, r) 1. if p < r 2. q = Randomized-Partition(A, p, r) 3. Randomized-Quicksort(A, p, q-1) 4. Randomized-Quicksort(A, q+1, r)

Quicksort with Good Average Running Time Randomization of Quicksort stops any specific type of array from causing worst- case behavior. For example, an already-sorted array causes worst-case behavior in non-randomized Quicksort but not in Randomized-Quicksort.

Average-case Analysis of Randomized-Quicksort Rename elements of A as z 1, z 2, …, z n and define the set Z ij to be the set of elements between z i and z j, inclusive. Each pair of elements is compared at most once, because elements are compared only to the pivot and the pivot is not used again. Let X ij = I{ z i is compared to z j } Since each pair is compared at most once, the total # comparisons= Taking the expectations of both sides, use L.5.1 and LoE: But what is Pr{z i is compared to z j }?

Average-case Analysis of Randomized-Quicksort Observations: Numbers in separate partitions are never compared. If the pivot is not z i or z j, these elements are never compared. The probability that z i is compared to z j is the probability that either one is chosen as the pivot. There are j – i + 1 elements and the probability that any particular one is chosen as the pivot is 1/(j – i + 1) Thus Pr{z i is compared to z j } = Pr{z i is the pivot} + Pr{z j is the pivot} = 2/(j-i+1). Substituting this for E[X] and letting k = j - i: So the expected running time of Randomized-Quicksort is O(nlgn) when elements in A are distinct.