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234 Chapter 4: Algorithms
26. (b) Given: 20, 47, 15, 8, 9, 4, 40, 30, 12, 17.
Here, two-way merge sort is used, so a group of
two is taken at once. The second pass is shown as
follows:
8, 15, 20, 47
12, 17
4, 9, 30, 40
20, 47
8, 45
4, 9
30, 40
12, 17
20, 47
15, 8
9, 4
40, 30
12, 17
After second pass of the algorithm element, the
order is 8, 15, 20, 47, 4, 9, 30, 40, 12, 17.
27. (c) Randomized quick sort worst-case complexity
is O(n2 ); when all the elements are same, it took
worst time.
28. (d)
29. (b) Look at the following figure:
15
3
30. (a) Bellman-Ford algorithm is used to find all-pair
shortest distances in a graph, and it is a dynamic
programming technique.
31. (b) Merge sort took O(n log n) to sort a list of element. Binary search time complexity is O(log n).
Insertion sort worst-case running time complexity
is O(n2 ) .
32. (c) d(r, u) and d(r, v) will be equal when u and
v are at same level; otherwise, d(r, u) will be less
than d(r, v).
33. (d) In an undirected graph, if it is a complete
graph, then it has n(n −1)/2 edges. We can choose
to have (or not have) any of the n(n −1)/2 edges.
So, the total number of undirected graphs with n
vertices is 2(n(n−1)/2).
34. (c) Depth-first search uses stack for storing values;
queue is used by breadth-first search. Heap is used
for sorting the set of elements.
50
62
58
20
5
The number of nodes in the left subtree and the
right subtree of the root, respectively, is (7, 4).
Therefore, option (b) is correct.
37
8
91
60
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Chapter 4.indd 234
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UNIT V: THEORY OF COMPUTATION
LAST SIX YEARS' GATE ANALYSIS
7
Number of questions
6
5
Marks 1
Marks 2
Total number of questions
4
3
2
1
0
2015 2014 2013 2012 2011 2010
Concepts on which questions were asked in the previous six years
Chapter 5.indd 235
Year
Concepts
2015
Turing machine, Regular expression and languages, DFA,
Context free grammar, Recursive language, NDPDA
2014
Regular languages, DFA, RE and REC
2013
Regular languages, DFA, CFL, CFG
2012
Regular languages, DFA and NFA, CFL
2011
Power of DFA, NFA and TM, Regular language, PDA
2010
Regular expression, Recursive language, LR and LL, CFL
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Chapter 5.indd 236
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CHAPTER 5
THEORY OF COMPUTATION
Syllabus: Regular languages and finite automata, context-free languages and pushdown automata, recursively
enumerable sets and Turing machines, undecidability.
5.1 INTRODUCTION
Theory of computation is a branch of computer science
and mathematics that studies whether a problem can
be solved using an algorithm on a model of computation. This branch also studies that how efficiently that
problem can be solved. This deals with two types of
theories. First is computability theory, deals with up
to which extent problem can be solved. Second is complexity theory, which deals with efficiency of solution.
In this finite automata, push down automata, Turing
machines, regular expressions and various grammars will
be discussed.
Automata theory is the study of abstract machines (or
more appropriately, abstract “mathematical” machines
or systems) and the computational problems that can be
solved using these machines. These abstract machines
are called automata. Computability theory deals with
Chapter 5.indd 237
the question of the extent to which a problem is solvable on a computer, whereas complexity theory considers not only whether a problem can be solved at all on
a computer, but also how efficiently it can be solved.
Two major aspects are considered in this—time complexity and space complexity.
Basic introduction and various technical terms for
understanding of concept are described in the following
section.
5.2 FINITE AUTOMATA
There are several things to be done in the designing and
implementation of an automatic machine, but the most
important question is that, what will be the behaviour
of the machine? “Theory of Computation” is the subject
which solves this purpose.
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