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Merge Sort

As a typical DnC algorithm, MergeSort tries to solve sorting problem in a recursive, distributed way. Its overall algorithm outperforms the InsertionSort and canonical BubbleSort algorithms.

Fundamental Ideas

MergeSort algorithm takes the unsorted original array and split it into С number of sub-arrays, feeding them individually into the algorithm itself for the same sorting purpose.

Then, resolve each sub-problem with the defined minimal length of sub-array. After recursive calls in each layer of division, combining the results of each subroutine to generate the final sorting solution.

Note: the task's aim might varies with scenarios. e.g. a bulk of sorted arrays reside in memory has to merge into an array with randomly filled elements; therefore, the actual implementation is not definite.

Pseudocode

Though the abstract details are the same, the top-down MergeSort and bottom-up MergeSort are two different implementations of MergeSort. The first one is suitable for education for its simplicity while the second one has a slightly better performance and widely adopted in many libraries.

Top-down Approach

It is a common practice to use such a recursive approach:

TOP_DOWN_MERGE_SORT(array, low, high, new_array)
  if high - low < 2
    return

  middle := (low + high) / 2
  TOP_DOWN_MERGE_SORT(array, low, middle, new_array)
  TOP_DOWN_MERGE_SORT(array, middle, high, new_array)

  TOP_DOWN_MERGE(array, low, middle, high, new_array)

  MERGE(array, low, middle, high, new_array)

Bottom-up Approach

rather than a recursive method in above approach, using a iterative method:

BOTTOM_UP_MERGE_SORT(array, new_array)
  for width := 1, width < length(array)
    for i := 0, i < length(array)
      MERGE(array, i, min(i+width, n), min(i+2*width, n), new_array)
      i := i + 2 * width
    width := width * 2

and the universal MERGE step:

MERGE(array, low, middle, high, new_array)
  i := low
  j := middle
  for k in range (high - low)
    if i < middle and array[i] < array[j]
      new_array[k] = array[i]
      i := i + 1
    else if j < high and array[j] < array[i]
      new_array[k] = array[j]
      j := j + 1

Algorithm Analysis

MergeSort is a stable sorting scheme that has average and worst-case running times of Ο(n ⋅ log(n)) regardless of the inputs.

It is simple to prove the running complexity by master method Τ(n) = 2 Τ(n/2) + Θ(n)

Additional References

  1. Combining MergeSort and InsertionSort: https://cs.stackexchange.com/questions/68179/combining-merge-sort-and-insertion-sort