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Merge Sort Merging

Merge Sort is a divide and conquer algorithm. It consists of two parts:

1) splitting the original list into smaller sorted lists recursively until there is only 1 element in the list, 2) merging back the presorted 1-element lists into 2-element lists, 4-element lists, and so on recursively.

The merging portion is iterative and takes 2 sublists. The first element of the left sublist is compared to the first element of the right sublist. If it is smaller, it is added to a new sorted list, and removed from the left sublist. If it is bigger, the first element of the right sublist is added instead to the sorted list and then removed from the right sublist. This is repeated until either the left or right sublist is empty. The remaining non-empty sublist is appended to the sorted list.

Big-O Runtime for Merge Sort

The Merge Sort algorithm is divided into two parts. The first part repeatedly splits the input list into smaller lists to eventually produce single-element lists. The best, worst and average runtime for this part is Θ(log N). The second part repeatedly merges and sorts the single-element lists to twice its size until the original input size is achieved. The best, worst and average runtime for this part is Θ(N). Therefore, the combined runtime is Θ(N log N).

Merge Sort Implementation in Python

We can implement the Merge Sort algorithm in Python using two functions, merge_sort(lst), the main function and merge(left, right), a helper function.

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Merge Sort: Conceptual
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  1. 1
    Merge sort is a sorting algorithm created by John von Neumann in 1945. Merge sort’s “killer app” was the strategy that breaks the list-to-be-sorted into smaller parts, sometimes called a _divide-an…
  2. 2
    Merge sorting takes two steps: splitting the data into “runs” or smaller components, and the re-combining those runs into sorted lists (the “merge”). When splitting the data, we divide the input t…
  3. 3
    When merging larger pre-sorted lists, we build the list similarly to how we did with single-element lists. Let’s call the two lists left and right. Both left and right are already sorted. We want…
  4. 4
    Merge sort was unique for its time in that the best, worst, and average time complexity are all the same: Θ(N*log(N)). This means an almost-sorted list will take the same amount of time as a comple…
  1. 1
    What is sorted by a sort? A sort takes in a list of some data. The data can be words that we want to sort in dictionary order, or people we want to sort by birth date, or really anything else that …
  2. 2
    How do we break up the data in a merge sort? We split it in half until there’s no more data to split. Our first step is to break down all of the items of the list into their own list.
  3. 3
    Our merge_sort() function so far only separates the input list into many different parts — pretty much the opposite of what you’d expect a merge sort to do. To actually perform the merging, w…
  4. 4
    Now we need to build out our result list. When we’re merging our lists together, we’re creating ordered lists that combine the elements of two lists.
  5. 5
    Since we’ve only technically depleted one of our two inputs to merge(), we want to add in the rest of the values to finish off our merge() function and return the sorted list.
  6. 6
    Let’s update our merge_sort() function so that it returns a sorted list finally!
  7. 7
    We’ve written our merge sort! The whole sort takes up two functions: merge_sort() which is called recursively breaks down an input list to smaller, more manageable pieces. merge() which is a help…

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