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A kway merge operation. Suppose you have ksorted arrays, each with nelements, and you want to combine them into a single sorted array ofkn elements.

(a)Here鈥檚 one strategy: Using the merge procedure from Section 2.3, merge the first two arrays, then merge in the third, then merge in the fourth, and so on. What is the time complexity of this algorithm, in terms of kand n?

(b) Give a more efficient solution to this problem, using divide-and-conquer.

Short Answer

Expert verified

(a) Time complexity:O(k2n)

(b) Time complexity:O(knlogk)

Step by step solution

01

Introduction to array

There are different kinds of Array: 1,2,3 -dimensional and multi-dimensional arrays. In the above question there will be sorted array technique through we can combine single sorted array in knelements. To check that see below detail related to combine theory of array.

02

Step 2:Give the time complexity of the given algorithm.

(a)

Combine each of thek arrays with the preceding array one after the other, for each array containing 1to nentries.

Suppose that merging the array takes linear time.

Originally, the first and second arrays are combined, with each array having a size of n.

As a result, it takesnn time, which equals 2n. The next array of sizen is then combined with both the earlier merged array2n .

As a result, merging three arrays takes3n time. It is then combined with the fourth array, which takes4n time.

As a result, the total time required to combine the entire array is

Timetaken=O(2n+3n++kn)=O(k2n)

Therefore, thetime complexity used to merge array is O(k2n).

03

                                                                                                                     Step 3:Efficient solution to perform k-way merge operation:

(b)

To begin, all narrays get separated into two equal groups, each withk2 arrays.

After that, every format's arrays were recursive combined one by one, and both sets merging array were merged together.

The issue is solved by breaking it into two smaller problems of size k2and combining them in a constant time nk.

ThusT(k),can be expressed as,

T(k)=2T(k2)+O(nk)鈥︹赌(1)

Consider the master theorem for the following recurrence equation,

T(k)=aT(kb)+kc鈥︹赌(2)

CompareEquations (1) and (2),kc equalsnk ,cequals 1,bequals2and aequals2 . If,c=logba then perhaps the running speed is, according to master's thesis.

T(k)=O(kclogk)鈥︹赌(3)

Here, the value of c=1and the value oflogba=log22=1which is equal to .Thus, substitute kcasnk in Equation (3). So, the running time of algorithm is,

T(k)=O(nklogk)

Therefore, the time complexity of the algorithm in terms of kand n is T(k)=O(nklogk).

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Most popular questions from this chapter

An array A[1...n] is said to have a majority element if more than half of its entries are the same. Given an array, the task is to design an efficient algorithm to tell whether the array has a majority element, and, if so, to find that element. The elements of the array are not necessarily from some ordered domain like the integers, and so there can be no comparisons of the form 鈥 is A[i]>A[j] ?鈥. (Think of the array elements as GIF files, say.) However you can answer questions of the form: 鈥渋s ..?鈥 in constant time.

(a) Show how to solve this problem in O(nlogn) time. (Hint: Split the array A into two arrays A1and A2of half the size. Does knowing the majority elements of A1and A2help you figure out the majority element of A? If so, you can use a divide-and-conquer approach.)

(b) Can you give a linear-time algorithm? (Hint: Here鈥檚 another divide-and-conquer approach:鈥 Pair up the elements of A arbitrarily, to get n/2 pairs鈥 Look at each pair: if the two elements are different, discard both of them; if they are the same, keep just one of them . Show that after this procedure there are at most n/2 elements left, and that they have a majority element if A does.)

In Section 1.2.3, we studied Euclid鈥檚 algorithm for computing the greatest common divisor (gcd) of two positive integers: the largest integer which divides them both. Here we will look at an alternative algorithm based on divide-and-conquer.

(a) Show that the following rule is true.

gcd(a,b)={2gcd(a2,b2)ifa,bareevengcd(ab2)ifaisodd,bisevengcd(a-b2,b)ifa,bareodd

(b) Give an efficient divide-and-conquer algorithm for greatest common divisor.

(c) How does the efficiency of your algorithm compare to Euclid鈥檚 algorithm if a and b are n-bit -bit integers? (In particular, since n might be large you cannot assume that basic arithmetic operations like addition take constant time.)

This problem illustrates how to do the Fourier Transform (FT) in modular arithmetic, for example, modulo .(a) There is a number such that all the powers ,2,...,6 are distinct (modulo ). Find this role="math" localid="1659339882657" , and show that +2+...+6=0. (Interestingly, for any prime modulus there is such a number.)

(b) Using the matrix form of the FT, produce the transform of the sequence (0,1,1,1,5,2) modulo 7; that is, multiply this vector by the matrix M6(), for the value of you found earlier. In the matrix multiplication, all calculations should be performed modulo 7.

(c) Write down the matrix necessary to perform the inverse FT. Show that multiplying by this matrix returns the original sequence. (Again all arithmetic should be performed modulo 7.)

(d) Now show how to multiply the polynomials and using the FT modulo 7.

Practice with polynomial multiplication by FFT.

(a) Suppose that you want to multiply the two polynomials x + 1 and x2+1using the FFT. Choose an appropriate power of two, find the FFT of the two sequences, multiply the results component wise, and compute the inverse FFT to get the final result.

(b) Repeat for the pair of polynomials 1+x+2x2and 2 + 3x.

Consider the task of searching a sorted array A[1,,n] for a given element x: a task we usually perform by binary search in time O(logn) . Show that any algorithm that accesses the array only via comparisons (that is, by asking questions of the form 鈥渋s A[i]z 0?鈥), must take (logn) steps.

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