Merge Sort: What it is and Algorithm?

Merge Sort
Merge Sort

Merge sort is an effective sorting algorithm based on the comparison. The algorithm is based on “Divide and conquer,” meaning the initial collection will be split into smaller pieces in this situation. This concept can be introduced recursively so that the collection is split into single components and then rebuilt into a sorted set. The re-construction of the algorithm merging portion is the phase where the components are ordered. In each phase of merge sort, the components are placed in the positions they have sorted, in the sub-collection. Distributing the authentic collection into its elements may lead to the formation of additional elements which are essentially not required. The handling of the sub-collections may be in place so that distinct collections are not required. In this article, our experts from Assignment Help will explain to you about merge sort with the help of examples and infographics.

There are different variants that exist regarding merge sort. These variants are mainly worried about decreasing the complexity of storage and the price of copying.

A simple solution to minimize the space of overhead to n/2 is to keep left and right of the array as a structure which seems to be combined but are actually not. Then into the temporary or cached space, a copy of the left part m is made and then the routine of the merge is directed to place the output of the merged sort in m. In order to make use of only one allocation in the memory, it is good to give temporary room outside the merge routine with this variant. As the last couple of lines before the return result declaration becomes superfluous, the unnecessary copying listed above is also mitigated.

When applied on arrays, one drawback of merge type is its O(n) requirement of memory for its operation. Several variants were suggested in-place:

  • There is an algorithm that needs a steady quantity of working memory: sufficient storage space to hold an input array element, and extra space to hold O(1) pointers in the input array. These algorithms are said to be not stable but yet they are able to achieve an O(n log n) time in spite of being unstable.
  • There have been several attempts to create an in-place merge algorithm that can be combined with a conventional merge type (bottom-up or top-down) to establish an in-place merge type. In this case, in-place would mean the taking off the logarithmic space of the stacks, since the standard type of merge requires that amount of space for its own use. Stable merging is possible with a constant amount of scratch space in O(n log n) time, but yet again this algorithm is also complicated and has high factors of consistency: merging lengths n and m can take 5n + 12 m + o(m) movements. This factor of high constant and complex nature of the algorithm has been produced easier to comprehend and simpler.  A simple linear time algorithm known as in-place practical merging is implemented to merge a sorted list using a set quantity of extra space. It will be helping in merging the linear time and at the same time providing a continues space allocation.
  • An option to reducing copying to various lists is to combine a fresh data field with each key (keys are called the elements in m). This field will be used to connect the keys and any related data in a sorted list (the record is called a key and its related data). Then the merging of the sorted lists takes place by altering the values of the link; there is no need to move documents at all. A field containing only a connection will usually be larger than a whole record, so less room will be used as well. This is a normal method of sorting, not limited to sorting done through the use of merge sort.

Natural merge sort

A natural type of merge is comparable to a kind of bottom-up merge except that any naturally occurring runs (sort sequences) are utilized in the input. Both bitonic, as well as monotonic runs, can be utilized, with lists being convenient data structures (use as LIFO or FIFO). The starting point assumes that each run is one item long in the bottom-up merge type. In practice, there will be many brief runs of random input information that just occur to be sorted. In the typical situation, because there are fewer runs to merge, the natural merge type may not need as many passes. In the best case, the input is already sorted (i.e. single run), so it only needs to make one pass through the data by the natural merge sort. Long natural runs are present in many practical instances, which is why natural merge type is utilized. Our experts from assignment help will now explain to you about the


The process of merge sort involves three parts

  • Dividing the issue into a number of smaller subproblems of the same issue.
  • Conquering the subproblems by continuously solving them. Solve the subproblems as basic cases if they are small enough.
  • Combine the subproblem alternatives into the initial issue solution.
Process of Merge Short
Process of Merge Short

Performance analysis

The merge sort algorithm output is very remarkable O(n log(n)). It is suggested to use a default library. Most languages nowadays use a better performing algorithm as the default sorting alternative in a true world implementation. In Java, there is a sort technique in the Util package, in the Arrays class. dual-pivot Quicksort, which conducts O(n log(n)) time complexity, is implemented based on the documentation.

According to the below Java code, in the function mergeSort, only two statements can create extra space. One is the centre, which requires the complexity of O(1) space. The other is the merge feature, which due to the presence of a temporary array(tem) requires O(n) space complexity. The algorithm is also a deep-first algorithm. At some stage, there is only one merge feature that can be performed. Hence, the complexity of the room here is O(n).

The median and worst-case performance of Merge Sorting n objects is O(n log n). Considering time of the merge sort for a length n list while it is in a running state is T(n), then the recurrence T(n)= 2T(n/2)+n follows from the algorithm definition which asks to put the algorithm to half the size of the two lists of the initial list and then the n steps made to merge the two output lists are added. The closed-form follows from the dividing-and conquering recurrences master theorem.

As multilevel memory hierarchies are used, the location of reference can be of paramount significance in software optimization on contemporary pcs. Some of the existing algorithms of merge sort which are mostly aware of the cache memory of the machine are introduced, whose tasks are notably selected to reduce the movements of the page that take place from the cache machine as well as outside the machine. For example, when subarrays of size S are reached, the merge sort having the tiled algorithm will immediately stop to divide the subarrays, where S is said to be the amount of cache memory that the machine or the CPU can hold. In order to prevent the memory from getting wrapped or no longer accessible, a sorting algorithm such as the insertion sort is used in this place first in order to have no cache memory issues and then with the mode of standard recursion, merge sort is then finally performed over it in order to get it sorted  in the given manner be ascending or descending. On computers that profit from cache optimization, this algorithm has proved better efficiency.

Merge Sort Process
Merge Sort Performance Analysis
Merge Sort Performance Analysis

Implementing merge sort

The function of merge sort should sort the subarray array [p. r] recursively. It means after calling merge sort on (array, p, r), the components should be sorted in ascending order from index p to index r of the array.

The algorithm of merge sort explains that: –

  • It is essential to realize that sorting need be performed of the size of the sub array is 0 and 1. Therefore, nothing is needed to be done.
  • Otherwise, to sort the subarray, merge type utilizes divide-and-conquer.

To combine the sorted sub-arrays array [p. q] and array [q+1.. r], use merge (array, p, q, r).

Uncomment the Program.assertEqual function at the bottom once implemented to verify the passing of the test assertion.

Merge Sort Implementation
Merge Sort Implementation

Important characteristics

  • In order to sort linked lists, Merge Sort is helpful.
  • Merge Sort is a stable sort, meaning that the same component in an array maintains its initial positions in relation to each other.
  • Merge sort overall time complexity is O(nLogn). It is more efficient because the runtime is also O(nlogn).
  • In the worst case, the space complexity of the Merge sort is O(n). This implies that for the last information sets this algorithm requires a lot of room and can slow down activities.

As multilevel memory hierarchies are used, the location of reference can be of paramount significance in software optimization on contemporary pcs. Cache-aware versions of the merge sort algorithm were proposed, whose operations were specifically selected to minimize the movement of pages to and from the memory cache of a machine. For example, when subarrays of size S are reached, the tiled merge sort algorithm stops dividing the subarrays, where S is the number of data items that fit into the cache of a CPU. Each of these subarrays is sorted with an in-place sorting algorithm, such as the type of insertion, to discourage memory swaps, and then the normal sort of merge is completed in the standard recursive mode. This algorithm has shown better performance on computers benefiting from cache optimization. It is proposed an alternative type of merge using the continuous extra room. This algorithm was subsequently refined. Many internal sorting apps also use a type of parallel sorting where the input is divided up to a greater amount of sub-lists, ideally, a number for which merging still makes the presently processed collection of websites fit in the primary memory.

Merge sort and other sorting techniques

While heapsort has the same time boundaries as merge sort, it requires only 1 auxiliary space instead of the n merge type. Effective quicksort implementations usually outperform merge sort on typical contemporary architectures to sort RAM-based arrays. On the other side, merge sort is a stable type and is more effective in managing slow sequential media access. Merge sort is often used in linked list and is said to be the best option for it: in this scenario, a merge sort is comparatively simple to execute in such a manner that it needs only some (1) additional room, and the slow random-access performance of a linked list makes some other algorithms (such as quicksort) perform badly and others (such as heapsort) impossible. The array is divided into only 2 halves (i.e. n/2) in the merge type. Whereas the array is divided into any proportion in the event of a quick sort. There is no compulsion to quickly split the range of components into equal components. Merge sort is able to work with any type of data set which is available without having any constraint on the size of the data set, whereas on the other hand, quick sort cannot work with large amount data sets

Examples of the mergeSort program in Java

Merge sort is the sorting technique that follows the approach of dividing and then conquering. For instance, think of an Array called A having n number of elements in it. The algorithm of this sorting theme would be processing the elements in the following 3 steps.

  • If it is found out that the array A contains 0 or only 1 element, then it is already said to be sorted in the given order. If it is not sorted then it would divide the array into two sub-arrays of equal parts.
  • The conquering part involves sorting the two arrays so made in the previous step by the help of merge sort.
  • Finally, all the sub-arrays are merged into a single and final file which is sorted in the given manner and thereby maintaining the order of the given array elements.





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