Merge Sort Algorithm

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Questions and Answers

Merge Sort is a sorting algorithm that follows the ______-and-conquer paradigm.

divide

In Merge Sort, the unsorted list is divided into 'n' sub-lists, each containing ______ element.

one

The key operation in Merge Sort is the ______ of two sorted sub-lists.

merging

Merge Sort is a ______ sorting algorithm, meaning that it maintains the relative order of equal elements.

<p>stable</p> Signup and view all the answers

Merge Sort has a consistent performance with a time complexity of $O(n log n)$ in the ______, average, and best cases.

<p>worst</p> Signup and view all the answers

Merge Sort can efficiently handle large datasets by reading and writing smaller ______ of data at a time, making it well-suited for external sorting scenarios.

<p>chunks</p> Signup and view all the answers

Merge Sort can be easily ______, taking advantage of multiple processors or cores to enhance performance.

<p>parallelized</p> Signup and view all the answers

Merge Sort has a straightforward and modular ______, making it easier to understand and maintain.

<p>implementation</p> Signup and view all the answers

The MergeSort repeatedly divides the array into two ______ until it tries to perform MergeSort on a subarray of size 1.

<p>halves</p> Signup and view all the answers

The merge step resolves the problem of merging two sorted ______ to build one large sorted list.

<p>lists</p> Signup and view all the answers

The algorithm maintains three ______, one for each of the two arrays and one for the final sorted array.

<p>pointers</p> Signup and view all the answers

The merge function creates ______ of the subarrays $L + A[p..q]$ and $M + A[q+1..r]$.

<p>copies</p> Signup and view all the answers

Until we reach the end of either L or M, pick the ______ among the elements from L and M and place them in the correct position at $A[p..q]$.

<p>larger</p> Signup and view all the answers

Before merging, pick up the remaining elements after running out of elements in either L or M and put in ______.

<p>A[p..q]</p> Signup and view all the answers

In the merge function, the current index of L is maintained by i, starting at ______.

<p>1</p> Signup and view all the answers

In the Java code, n1 is calculated as qp + 1, representing the size of subarray ______.

<p>L</p> Signup and view all the answers

The condition if (L[i] <= M[j]) in the merge function checks if the element in L is less than or equal to the element in ______.

<p>M</p> Signup and view all the answers

The mergeSort function recursively calls itself with the subarrays arr, l, m and arr, m + 1, ______.

<p>r</p> Signup and view all the answers

The space complexity of Merge Sort is O(______).

<p>n</p> Signup and view all the answers

Merge Sort is a ______ sorting algorithm according to the additional notes.

<p>yes</p> Signup and view all the answers

Flashcards

What is Merge Sort?

A sorting algorithm that divides the unsorted list into 'n' sub-lists, each containing one element, and then repeatedly merging sub-lists to produce new sorted sub-lists until there is only one sub-list remaining, which is the sorted list.

What is a stable sorting algorithm?

It maintains the relative order of equal elements in the sorted output.

What is the time complexity of Merge Sort?

Merge Sort has a consistent and predictable performance with a time complexity of O(n log n) in the worst, average, and best cases.

When is Merge Sort useful for external sorting?

Merge Sort is particularly well-suited for external sorting scenarios where data is too large to fit into memory. It can efficiently handle large datasets by reading and writing smaller chunks of data at a time.

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Does Merge Sort allow parallelization?

Merge Sort can be easily parallelized, taking advantage of multiple processors or cores to enhance performance.

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Is Merge Sort memory-efficient?

Merge Sort typically uses a consistent amount of additional memory regardless of the input data size.

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What is main idea of Merge Sort?

Divide the array into two subarrays, sort them and merge them

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How does MergeSort divide?

Repeatedly divides the array into two halves until we reach a base case of subarray of size 1.

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What does merging mean in MergeSort?

Merging two sorted lists (arrays) to build one large sorted list (array).

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What subarrays created in merge function?

L + A[p..q] and M + A[q+1..r]

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What are the three pointers used in Merge Sort?

i maintains current index of L, starting at 1. j maintains current index of M, starting at 1. k maintains the current index of A[p..q], starting at p.

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Space complexity of Merge Sort

O(n)

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Study Notes

  • Merge Sort is a sorting algorithm that uses the divide-and-conquer paradigm.
  • It divides an unsorted list into n sub-lists, each with one element.
  • It repeatedly merges sub-lists to create new sorted sub-lists until only one sorted sub-list remains.

Merge Sort Algorithm Steps

  • Divide: Split the unsorted list into n sub-lists, each containing one element.
  • Conquer: Repeatedly merge sub-lists into sorted sub-lists until one remains.
  • Merge: Combine two sub-lists into a single sorted sub-list, repeating until the final sorted list is achieved.
  • The merging of two sorted sub-lists is the key operation.

Advantages

  • Stability: Merge Sort is stable, preserving the relative order of equal elements in the output.
  • Predictable Performance: It has a time complexity of O(n log n) in all cases.
  • External Sorting: Well-suited for external sorting where data is too large for memory.
  • Parallelization: Can be easily parallelized for performance enhancement.
  • Consistent Memory Usage: Uses a consistent amount of additional memory.
  • Ease of Implementation: Straightforward and modular implementation.

Considerations

  • Other algorithms like Quicksort, Heapsort, or insertion sort may be more appropriate depending on specific data requirements.
  • Quicksort may be faster for small or pre-sorted datasets.

Decomposition

  • MergeSort repeatedly divides the array into two halves until reaching a subarray of size 1.
  • Achieved through recursion until it stops.

Merging Process

  • The merge step involves merging two sorted lists (arrays) into one large sorted list (array).
  • The algorithm uses three pointers: one for each array and one for the final sorted array.
  • It compares elements from both arrays and copies the smaller element into the sorted array, moving the pointer accordingly.
  • Remaining elements from the non-empty array are then copied.

Code Implementation

  • Task: Merge two subarrays A[p..q] and A[q+1..r] into a sorted array A[p..r].
  • Inputs: A, p, q, and r.
  • Steps:
    • Create copies of subarrays L (A[p..q]) and M (A[q+1..r]).
    • Initialize three pointers i, j, and k for L, M, and A[p..q], respectively.
    • Select larger elements from either L or M and place them in the correct position in A[p..q] until one of them runs out of elements.
    • Transfer the remaining elements into A[p..q].

Time and Space Complexity

  • Time Complexity:
    • Best: O(n*log n)
    • Worst: O(n*log n)
    • Average: O(n*log n)
  • Space Complexity: O(n)
  • Stable: Yes

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