# algorithms

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ALGORITHMS
1   What is backtracking

A) For some problems solution is expressible as an N- table (X1,X2-----------Xn) where the
Xi are chosen from some finite set Si. The brute force method would yeiled all these n tuples
, evaluate each one with the criterion function P and same those which yeid the
optimum.Backtracking is             a which is used in superior to brute force method. Its basic
idea is to buid up the solution vector one component at a time and to use modified criterion
functions Pi (x1,x2---------xi), a bounding function to test whether the vector being formed
has any chance of success. The major advantage of this method is if it is realized that the
partial vector (X1,X2---------Xi) can in no way lead to an optimal solution, then m I+1, …mn
possible test vectors can be ignored entirely . The problems which uses backtracking requires
that they have to satisfy the constraints divided into two categories.

1) Explicit                    2) Implicit

Explicit constraints are used that restrict each Xi to take on values only from a given set.
Implicit constraints are rules that determined which of the tuples in the solution spare of I
satisfy the criterion function. Thus these describe the way in which the Xi must rebate to each
other.

2) Which of the sorting algorithms uses Divide and conquer strategy

A) Given a function to compute on N inputs the divide and conquer strategy suggests splitting
the inputs into k distinct subsets, 1<K=n, yeliding k sub problems. These sub problems must
be solved and then a method must be found to combine sub solutions into a solution of the
whole. If the sub problems are still relatively large, then the divide and conquer strategy can
possibly be re applied.For problems like binary search, merge sort, quicksort divide and
conquer stratify can be used.

3) What is branch and bound strategy

A) A branch and bound method searches a state stat the using any search mechanism in which
all the children of the E-node are generated before another node becomes the E node. Three
common strategis are FIF0,LIFO and LC. A cost function C( ) such that C (X)>= C(x) is used
to provide lower bounds on solution obtainable form any node X. If u is an upper bound on
the cost of a minimu cost solution of live nodes X with C-(x)>U may be killed as all answer
nodes rechable form X have cost c(x)>=c-(x)>U. In case an answer node with cost U has
already been reached then all live nodes with c(x)>= >U my be killed.

4) What is dynamic programming

A) A Dynamic programming is an algorithms design method that can be used when
the solution to a problem can be viewed as the result of a sequence of desiccions. We
could enumerate all decisions sequences and then pick out the best. The optimal
sequence decisions are obtainedo by making explic it appeal to the principle of
optimality.

The principle of optimality states that an optimal sequnec of decisions has the
property that what ever the initial state and desion are, the remaining must constitute
an optimal desicion sequnece, with regard to the state resulting from the first
decision..
5) what is Greedy Method

A) Most of the problems have N inputs and require us to obtain a subset that sales us some
constraints. Any subset that satifies these constraints is called a feasible solution. We need to
obtain a feasible solution that either maximises / minimizes a given objective function. A
feasible solution that does this is called an optimal solotio. The greedy method suggests that
one can devise an algorithms that works in stages considering one input at a time. At each
stage desicion is made regarding whether a particular input is in an optimal solution (using
some selection procudure). If the inclusion of the next input into the partially constructed
optimal solution will result in an unfeasible solution, the this this input is not added to the
partial solution. otherwise it is added. This version of greedy method is called subset
paradigm. For problems that do not call for the selection of an optimal subset in the greedy
method, we make decisions by considering the inputs in some order. Each decision is made
using an optimization criterion that can be computed using decision already made. This
version of greedy technique is called ordering paradegim.

6) What is time complexity?

A) Time complexity of algorithms

• When you need to write a program that needs to be reused many times other issues may
arise

– how much times does it take to run it?

– how much storage space do its variables use?

– how much network traffic does it generate?

• For large problems the running time is what really determines whether a program should be
used

• To measure the running time of a program, we – Select different sets of inputs that it should
be tested on (for benchmarking).

Such inputs may correspond to

• the easiest case of the problem that needs to be solved(best case)

• the hardest case of the problem that needs to be solved(worse case)

• a case that falls between these two extremes(average case)

• In many cases, the running time of a program depends on a particular type of input, not just
the size of the input

– the running time of a factorial program depends on the particular number whose factorial is
being sought because this determines the total number of multiplications that need to be
performed

– the running time of a search program may depend on whether the value being sought occurs
in the collection of items to be searched
• To assess the running time, we have to accept the idea that certain programming operations
take a fixed amount of time (independent of the input size):

– arithmetic operations (+, -, *, etc.)

– logical operations (and, or, not)

– comparison operations (==, <, >, etc.)

– array/vector indexing

– simple assignments (n = 2, etc.)
Notation   Name                    Example

constant                Determining if a number is even or odd

The find algorithm of Hopcroft and Ullman on a
iterated logarithmic
disjoint set

Finding an item in a sorted list with the binary
logarithmic
search algorithm

Deciding if n is prime with the AKS primality
polylogarithmic
test

fractional power        searching in a kd-tree

linear                  Finding an item in an unsorted list

linearithmic, loglinear,
Sorting a list with heapsort, computing a FFT
or quasilinear

Sorting a list with insertion sort, computing a
DFT

polynomial, sometimes Finding the shortest path on a weighted digraph
called algebraic      with the Floyd-Warshall algorithm

exponential,              Finding the (exact) solution to the traveling
sometimes          called salesman problem (under the assumption that P
geometric                 ≠ NP)

Determining if two logical statements are
factorial, sometimes equivalent [1], traveling salesman problem, or
called combinatorial any other NP Complete problem via brute-force
search

Finding a complete         set   of   associative-
double exponential
commutative unifiers [2]
Alternative definition
Mathematical
Notation        Definition
definition
asymptotic
upper bound

asymptotic
lower bound

asymptotically
tight bound

asymptotically
negligible

asymptotically
dominant

7) what are the different notations used to measure the time complexity

A)Related asymptotic notations: O, o, Ω, ω, Θ, Õ

Big O is the most commonly used asymptotic notation for comparing functions, although in
many cases Big O may be replaced with Θ for asymptotically tighter bounds (Theta). Here,
we define some related notations in terms of "big O":

8) How the efficiency of an algorithm is measured

A) Algorithmic efficiency

Efficiency is generally contained in two properties: speed, (the time it takes for an operation
to complete), and space, (the memory or non-volatile storage used up by the construct).
Optimization is the process of making code as efficient as possible, sometimes focusing on
space at the cost of speed, or vice versa.

The speed of an algorithm is measured in various ways. The most common method uses time
complexity to determine the Big –oh of an algorithm: often, it is possible to make an
algorithm faster at the expense of space. This is the case whenever you cache the result of an
expensive calculation rather than recalculating it on demand. The space of an algorithm is
actually two separate but related things. The first part is the space taken up by the compiled
executable on disk (or equivalent, depending on the hardware and language) by the
algorithm.
9) what is space complexity

A) Space complexity

The better the time complexity of an algorithm is, the faster the algorithm will carry out his
work in practice. Apart from time complexity, its space complexity is also important: This is
essentially the number of memory cells which an algorithm needs. A good algorithm keeps
this number as small as possible, too.

There is often a time-space-tradeoff involved in a problem, that is, it cannot be solved with
few computing time and low memory consumption. One then has to make a compromise and
to exchange computing time for memory consumption or vice versa, depending on which
algorithm one chooses and how one parameterizes it.

10) what are the best , average and worst case complexities of (selection , bubble
insertion ,quick , heap ,merge sorting methods)

A) Classification

   For typical sorting algorithms good behavior is O (n log n) and bad behavior is Ω(n2).
Ideal behavior for a sort is O(n). Sort algorithms which only use an abstract key
comparison operation always need at least Ω(n log n) comparisons on average.

List of sorting algorithms

In this table, n is the number of records to be sorted and k is the average length of the keys.
The columns "Best", "Average", and "Worst" give the time complexity in each case;
estimates that do not use k assume k to be constant. "Memory" denotes the amount of
auxiliary storage needed beyond that used by the list itself..
Name        Best      Average Worst             Memory Stable Method         Other notes

Bubble
O(n)      —           O(n2)         O(1)    Yes   Exchanging Times are for best variant
sort

Selection
O(n2)       O(n2)       O(n2)         O(1)    No    Selection
sort

Insertion             O(n     +                                              d is the number of
O(n)                    O(n2)         O(1)    Yes   Insertion
sort                  d)                                                     inversions, which is O(n2)

Merge       O(n    log O(n log O(n        log
O(n)    Yes   Merging
sort        n)         n)      n)

In-place
O(n       log O(n log O(n        log                                Memory is for in-place
merge                                           O(1)    Yes   Merging
n)            n)      n)                                            partition algorithm
sort

O(n    log O(n log O(n        log
Heapsort                                        O(1)    No    Selection
n)         n)      n)

O(n    log O(n log                                               Naive   variants    use    O(n)
Quicksort                      O(n2)            O(log n) No   Partitioning
n)         n)                                                    space

11) wat is an inplace and non-inplace algorithms

A) An algorithm which is not in-place is sometimes called not-in-place or out-of-placeAn
algorithm is sometimes informally called in-place as long as it overwrites its input with its
output. In reality this is not sufficient (as the case of quicksort demonstrates) nor is it
necessary; the output space may be constant, or may not even be counted, for example if the
output is to a stream. On the other hand, sometimes it may be more practical to count the
output space in determining whether an algorithm is in-place, such as in the first reverse
example below; this makes it difficult to strictly define in-place algorithms.

12) What is the order of an algorithm which perfrms matrix multiplication
2.807
A) The order of the matrix multiplication using strassens matrix multiplication is n
13) What is the best , average and worse case complexities of an algorithm

A)Time complexity in the best case measures the leaset amount of time that the algorithms
needs to solve an instance of the problem

Time complexity in the worse case measures the largest amount of time that the algorithms
needs to solve an instance of the problem

Time complexity in the average case measures the average amount of time that the algorithms
needs to solve an instance of the problem

14) what is the best case average case and worse time complexities of binary search and
linear search

A) Complexity of linear search for successful searches

Best case       Ө(1)

Average case Ө(log n)

Worse case Ө(log n)

Complexity of linear search for unsuccessful searches

Best case       Ө(logn)

Average case Ө(log n)

Worse case Ө(log n)

Complexity of binary search

Best case       Ө(logn)

Average case Ө(log n)

Worse case Ө(log n)

15) Which sorting algorithms has very high space complexity

A) Merge sort takes high space complexity

16) how many maximum comparisions are required o search a key in binary search tree

A) The number of element comparisions is atmost k for a successful is in the rane of (2k-1 ,2k)
for binary search tree. For unsuccessful search the element comparision are either K or k-1.
unsuccessful complexity is Ө(logn). For successful search the element comparision are either
K or k-1. unsuccessful complexity is O(logn)
17) what is a pass

A) It is an iteration or one time execution of a loop for n times

18)what is the best , average and worse case time complexity of the push operation of a
stack

A) It is O(1)

19) what is the best , average and worse case time complexity of the pop operation of a
stack

A) It is O(1)

20) what is the best , average and worse case time complexity of the insert and delete
operations of a queue

A) It is O(n) for insert and O(1) for delete operations

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 views: 40 posted: 5/12/2011 language: English pages: 9