DSC 40B – Theoretical Foundations of Data Science II


Problems tagged with "mergesort"

Problem #018

Tags: best and worst case, mergesort

True or False: the best case time complexity of mergesort is \(\Theta(n)\).

True False
Solution

False.

Problem #019

Tags: mergesort

Suppose mergesort is called on an array of size \(n\). In total, how many calls to merge are made over all calls to mergesort?

Solution

\(\Theta(n)\)

Problem #068

Tags: recurrence relations, mergesort

Consider the modification of mergesort shown below, where one of the recursive calls has been replaced by an in-place version of selection_sort. Recall that selection_sort takes \(\Theta(n^2)\) time.


def kinda_mergesort(arr):
    """Sort array in-place."""
    if len(arr) > 1:
        middle = math.floor(len(arr) / 2)
        left = arr[:middle]
        right = arr[middle:]
        mergesort(left)
        selection_sort(right)
        merge(left, right, arr)

What is the time complexity of kinda_mergesort?

Solution

\(\Theta(n^2)\)

Problem #081

Tags: recursion, mergesort

Consider the code for mergesort and merge where a new print-statement has been added to merge:



def mergesort(arr):
    """Sort array in-place."""
    if len(arr) > 1:
        middle = math.floor(len(arr) / 2)
        left = arr[:middle]
        right = arr[middle:]
        mergesort(left)
        mergesort(right)
        merge(left, right, arr)

def merge(left, right, out):
    """Merge sorted arrays, store in out."""
    left.append(float('inf'))
    right.append(float('inf'))
    left_ix = 0
    right_ix = 0

    for ix in range(len(out)):
        print("Here!") # <---- the newly-added line
        if left[left_ix] < right[right_ix]:
            out[ix] = left[left_ix]
            left_ix += 1
        else:
            out[ix] = right[right_ix]
            right_ix += 1

Suppose mergesort is called on an array of size \(n\). In total, how many times will "Here!" be printed?

Solution

\(\Theta(n \log n)\)