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Merge Sort Algorithm in Python

Merge Sort Algorithm in Python

Hello everyone, welcome back to Programming In Python. Here in this post am going to tell you how to implement Merge Sort Algorithm in Python. In the previous posts, I have said about Selection Sort and Bubble Sort, here this Merge sort is much more efficient than those two algorithms as their time complexity is O(n2) in the average case and here it is O(n log n) in the worst case, this says how efficient Merge sort is than Selection and Bubble sort.

Merge sort algorithm is a divide and conquer algorithm, where the main unsorted list is divided into 2 halves, and each of the halves is again divided into 2 halves until only a single element is left in the list. Now merge these sublists by sorting them and finally, the main list is sorted.

You can also watch the video on YouTube here

Program on GitHub

See the animation below,

Merge Sort

By Swfung8Own work, CC BY-SA 3.0, Link

Time Complexity Of Merge Sort

Best Case O(n log n)
Average Case O(n log n)
Worst Case O(n log n)


  1. Divide the unsorted list into n sublists, until each list has only 1 element (a list of 1 element is considered sorted).
  2. Repeatedly merge sublists to produce new sorted sublists until there is only 1 sublist remaining. This will be the sorted list.

Merge Sort Algorithm – Code Visualization

Program on GitHub


__author__ = 'Avinash'

def merge_sort(sort_list):
    print("splitting", sort_list)
    if len(sort_list) > 1:

        mid = len(sort_list) // 2
        leftHalf = sort_list[:mid]
        rightHalf = sort_list[mid:]


        i = 0
        j = 0
        k = 0

        while i < len(leftHalf) and j < len(rightHalf):
            if leftHalf[i] < rightHalf[j]:
                sort_list[k] = leftHalf[i]
                i = i + 1
                sort_list[k] = rightHalf[j]
                j = j + 1
            k = k + 1

        while i < len(leftHalf):
            sort_list[k] = leftHalf[i]
            i = i + 1
            k = k + 1

        while j < len(rightHalf):
            sort_list[k] = rightHalf[j]
            j = j + 1
            k = k + 1

    print("merging...", sort_list)

lst = []

size = int(input("Enter size of the list: \t"))

for i in range(size):
    elements = int(input("Enter an element: \t"))


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Merge Sort Algorithm
Merge Sort Algorithm in Python

Merge Sort Algorithm
Merge Sort Algorithm

Program on GitHub

Also, feel free to look at other sorting algorithms like Bubble Sort or Selection Sort and searching algorithms like Linear Search and Binary Search.

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