November 05, 2018

Design and Analysis of Algorithms - Mid Sem Solution


Question 1A)
The time factor when determining the efficiency of algorithm is measured by
a) Counting microseconds
b) Counting the number of key operations
c) Counting the number of statements
d) Counting the kilobytes of algorithm

Answer: b
Justification: It is hardware and language independent , rest are dependent on hardware or on software

1B) The concept of order Big O is important because
a) It can be used to decide the best algorithm that solves a given problem
b) It determines the maximum size of a problem that can be solved in a given  amount of time
c) It is the lower bound of the growth rate of algorithm
d) Both A and B


Answer: a
Justification: We know that Big Oh notation is used in time complexity . the reason why we called Big Oh because it says "at any condition this is worst time that an algorithm could take " where as Big Omega says its is "best time that you can get , You cant get better than this.

1C) In worst case Quick Sort has order
A) O (n log n)            B. O (n2 /2)                C. O (log n)                         D. O (n2 /4)

Answer: A
Justification: In the worst case of quick sort has order O(n2). Quick sort is the quickest comparison-based sorting algorithm. It is very fast and requires less additional space, only O(n log n) space is required.

1D) Consider the following program segment. What is the Space complexity?
Begin
i=0;
S=0;
S=s+1;
Return s;
End

Answer: [If i,S considered as integers] => Space Complexity = 4 +(4*2)  = 12 bytes, where additional 4 bytes is for return value

To calculate the Space complexity of an algorithm, we need to follow the below instruction first:
It is the amount of memory used by the algorithm (including the input values to the algorithm) to execute and produce the result.

Space Complexity = Auxiliary Space + Input space

Calculating the Space Complexity:

For calculating the space complexity, we need to know the value of memory used by different type of datatype variables, which generally varies for different operating systems, but the method for calculating the space complexity remains the same.
TypeSize
bool, char, unsigned char, signed char1 byte
short, unsigned short,2 bytes
float, int, unsigned int, long, unsigned long4 bytes
double, long double, long long8 bytes
1E)  What is the time complexity of optimal binary search
(1) a)O(n) b) O(1) c)O(2n/2 d) O(n2)

Answer: d
Justification:
An optimal binary search tree is a binary search tree for which the nodes are arranged on levels such that the tree cost is minimum.

For Reference:
The complexity of linear search algorithm is
a) O(n)
b) O(log n)
c) O(n2)
d) O(n log n)

Answer: a
Justification: It refers to n values complexity in the algorithm which can be reduced by choosing the other algorithms.

The complexity of Bubble sort algorithm is
a) O(n)
b) O(log n)
c) O(n2)
d) O(n log n)

Answer: c
Justification: Bubble sort, is a simple sorting algorithm that works by repeatedly stepping through the list to be sorted, comparing each pair of adjacent items and swapping them if they are in the wrong order.

Question
Randomised algorithms are also called as __________________ and whose behavior is dependent  on  _______________ in decision making as part of its logic

Answer: probabilistic algorithm and randomness

Question:
Match the following    


Problem

Recurrence Equation
1
Binary Search 
A
tn – tn-1 = 1
2
Merge Sort
B
T(n)=2T(n/2)+n-1
3
Sequential Search
C
tn =  tn-1 + n
4
Factorial
D
T(n)=7T(n/2)+18(n/2)2
5
Strassen Matrix Multiplication
E
T(n)=T(n/2)+1
6
Selection Sort
F
T(n)=T(n/2)+n-1









Answer:

T(n) = Time Complexity
 

Problem

Recurrence Equation
1
Binary Search
E
T(n)=T(n/2)+1
2
Merge Sort
B
T(n)=2T(n/2)+n-1
3
Sequential Search
A
tn – tn-1 = 1
4
Factorial
F
T(n)=T(n/2)+n-1
5
Strassen Matrix Multiplication
D
T(n)=7T(n/2)+18(n/2)2
6
Selection Sort
C
tn =  tn-1 + n

Additional Reference:


7
Insertion sort
-
T(N) = T(N-1) + N-1
8
Tree traversal
-
T(n) = 2T(n/2) + 1
9
Quicksort
-
(n) = 2T(n/2) + n
10
Master Theorm
-
T(n)=aT(n/b)+f(n)












Tip:
Merge Sort: The merge sort algorithm deals with the problem of sorting a list of n elements. It is able to sort a list of n elements in O(n log n) run-time, which is considerably faster than insertion sort, which takes O(n2).
Merge sort uses a divide and conquer method:
1. If the length of the list is 1, the list is sorted. Return the list.
2. Otherwise, split the list in two (roughly) equal halves and then recursively merge sort the two halves
3. Merge the two sorted halves into one sorted list.

Master Theorem:

Generic equation: T(n) = aT(n/b) + f(n) where a >= 1 and b > 1

There are following three cases:
1. If f(n) < Logba then T(n) = Θ(nLogba)
2. If f(n) = Logba then T(n) = Θ(nLogbLog n)
3.If f(n) >  Logba then T(n) = Θ(f(n))
Question:
S = {A, B, C, B} ,  T = {B, D, C, A} Find the Longest Common  Sub-sequence of S and T with detailed procedure.

Solutions:
Before jumping to any solutions, we need to understand the logic first (Recurrence). Try to understand 3 key points below for further moving:

S = {A, B, C, B}, T = {B, D, C, A}
So we define Si, Tj to be the prefixes of S and T of length i and j respectively.

Let, C[i,j]=LCS(Si, Tj)

Recurrence Tip:
1 If i=0 and j=0 then value would be 0
2 If Si=Tj then value would be as C [i-1, j-1] + 1    [Meaning, Character matched -> apply formula]
3 If Si Tj then value would be as max{C [i, j-1], C [i-1, j] }     [Meaning, Not matched ->  take the max out of both]

Given: - String as S and T with the length of 4 and 4 respectively. m=4 and n=4 

1st Operation:
C (1, 1) = max { C (0,1) , C(1,0) } = 0            [Character, not matched - > Apply 3rd formula]
C (1, 2) = max { C (0,2) , C(1,1) } = 0
C (1, 3) = max { C (0,3) , C(1,2) } = 0
C (1, 4) = C (0, 3) + 1 = 0+1 = 1                     [Character matched -> apply 2nd formula]

2nd Operation:
C (2, 1) = C (1, 0) + 1 = 0 + 1 = 1  
C (2, 2) = max { C (1,2) , C(2,1) }  = 1
C (2, 3) = max { C (1,3) , C(2,2) }  = 1
C (2, 4) = max { C (1,4) , C(2,2) }  = 1 

3rd Operation:
C (3, 1) = max { C (2,1) , C(3,0) } = 1           
C (3, 2) = max { C (2,2) , C(3,1) } = 1
C (3, 3) = C(2,2) + 1 = 2
C (3, 4) = max { C (2,4) , C(3,3) } =2

4th Operation:
C (4, 1) = C (3, 0) + 1 = 0 + 1 = 1
C (4, 2) = max { C (3,2) , C(4,1) }  = 1
C (4, 3) = max { C (3,3) , C(4,2) }  = 2
C (4, 4) = max { C (3,4) , C(4,3) }  = 2

Finally we get the table as given below:

 

Finding the LCS:

* The last digit defines the length of our LCS.
* Follow the below table to get the LCS.


*The LCS is :  B C

Question:
Apply merge sort for the following list of elements: 6, 3, 7, 8, 2, 4, 5, 1 . Analyze  the time complexity of Merge Sort.

Solution:

Merge sort is a recursive algorithm that continually splits a list in half. If the list is empty or has one item, it is sorted by definition.  If the list has more than one item, we split the list and recursively invoke a merge sort on both halves.
Once the two halves are sorted, the fundamental operation, called a merge, is performed. Merging is the process of taking two smaller sorted lists and combining them together into a single, sorted.

Time Complexity : O(n1 + n2)

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