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Python | Merge sort Program

Alankrit Srivastava
Last Updated: Mar 6, 2023


When we are introduced to the world of programming, sorting problems are our frequent encounters. We learn many sorting approaches and algo for these types of problems. We will discuss an efficient sorting algo that you will use throughout your programming career.


This article will discuss merge sort in python. Merge sort is a sorting algorithm that uses a divide-and-conquer strategy to sort an array or list. It works by dividing the array into two equal parts, sorting each part separately, and then merging the sorted parts back together. We will be discussing merge sort in python from scratch.

“ Recommended Topic, Python Round Function

Divide and Conquer Approach

Divide and conquer is a strategy or technique used to solve problems by breaking them down into smaller, more manageable subproblems. The idea is to divide the problem into smaller parts that are easier to solve, then solve each part separately, and finally combine the solutions of the smaller parts to find the solution to the original problem. This strategy is used in many different areas, including algorithm design, computer science, mathematics, and more.

divide and conquer algo

The divide and conquer approach is often used to solve problems with a recursive structure, which means that the problem can be broken down into smaller subproblems similar to the original problem. This approach can be applied to a wide range of problems, such as sorting algorithms (e.g. merge sort, quick sort), recursive algorithms (e.g. the Fibonacci sequence, the Tower of Hanoi), and more complex problems, such as the closest pair of points problem, the travelling salesman problem and many more.

Divide and conquer algorithms generally have a time complexity of O(n log n). It makes them quite efficient, but it also depends on the problem.

Merge Sort in Python

Merge sort in python is a divide-and-conquer sorting algorithm. It works by repeatedly splitting a list of items in half until each half contains only one item. Then merging the halves back together in sorted order.


merge sort in python program example

The merge sort function first checks the base case where the array has a length of 1. If yes, then it returns the array. Otherwise, it divides the array into two parts and calls merge sort recursively on both parts. Finally, it merges both parts using the merge function, which takes two sorted arrays and returns a sorted array.

Pseudo code for Merge sort

procedure mergeSort(A: array of items, low, high)
    if (low < high) then
        mid = (low + high) / 2
        mergeSort(A, low, mid)
        mergeSort(A, mid + 1, high)
        merge(A, low, mid, high)

procedure merge(A: array of items, low, mid, high)
    create two arrays L[mid - low + 1] and R[high - mid]
    for i = 0 to mid - low do
        L[i] = A[low + i]
    for j = 0 to high - mid - 1 do
        R[j] = A[mid + j + 1]
    i = 0, j = 0, k = low
    while i < length(L) and j < length(R) do
        if L[i] <= R[j] then
            A[k] = L[i]
            i = i + 1
            A[k] = R[j]
            j = j + 1
        k = k + 1
    while i < length(L) do
        A[k] = L[i]
        i = i + 1
        k = k + 1
    while j < length(R) do
        A[k] = R[j]
        j = j + 1
        k = k + 1


The mergeSort function takes array A of items and the indices ‘low’ and ‘high’ of the portion of the array that should be sorted.

The function first checks if ‘low’ is less than ‘high’. If it is, the function calculates the middle index ‘mid’ of the current portion of the array and then recursively calls itself with the subarrays ‘A[low...mid]’ and ‘A[mid+1...high]’. This process continues until the entire array is divided into single-item subarrays.

When the recursive calls begin to return, the merge function is called to merge the sorted subarrays back together. The merge function creates two arrays, L and R, which will temporarily store the subarrays. The function then compares the first element of L and R, places the smaller of the two at the first position of the A array, and moves the pointer of that array.

Then it continues with the next elements of L and R until one of the subarrays is finished. Then it copies the remaining elements from the other array to the A array. The merge function is called repeatedly until the entire array is sorted.

Python Program

def merge_sort(arr):
    if len(arr) <= 1:
        return arr
    mid = len(arr) // 2
    left_half = arr[:mid]
    right_half = arr[mid:]
    left_half = merge_sort(left_half)
    right_half = merge_sort(right_half)
    return merge(left_half, right_half)

def merge(left, right):
    result = []
    i = 0
    j = 0
    while i < len(left) and j < len(right):
        if left[i] < right[j]:
            i += 1
            j += 1
    result += left[i:]
    result += right[j:]
    return result

# Example usage
arr = [3, 2, 1, 4, 5, 6, 7]
sorted_arr = merge_sort(arr)


output for program


In the above merge sort in python program, the merge_sort function takes in an array and recursively splits it into smaller arrays. It will be done until each array has only one element. Then the merge function is used to merge the sorted arrays back together. The merge function takes in two arrays and compares the first element of each array, adding the smaller element to the result and moving on to the next element until one of the arrays is exhausted. Then it adds the remaining elements of the other array to the result.

Time Complexity

The time complexity of the above merge sort in python program is O(n log n). Where n is the number of elements in the list.

Space Complexity 

The space complexity of the above merge sort in python program is O(n), where n is the number of elements in the list.

  • Additionally, it takes O(log n) recursive calls, which take up O(log n) stack space.
  • It requires additional memory to store the two sublists used in the merge operation. It takes up O(n) space. 
  • However, O(n + log n) is equivalent to O(n) space complexity.

Frequently Asked Questions

What is the algorithm of the sort() method in Python?

The sort() method in Python uses TimSort algo. It is a hybrid sorting algorithm that combines the best aspects of both merge sort and insertion sort. It was developed by Tim Peters and was first introduced in Python 2.3.

How do you sort code in Python?

In Python, there are several ways to sort a list of elements. The most common and easiest way to sort a list is by using the built-in sort() method. It sorts the elements in ascending order. The sort() method modifies the original list in place and returns None.

What is the difference between sort () and sorted () in Python?

The main difference between the two is that sort() modifies the original list and returns None. Whereas sorted() creates a new list and returns it. Also, the sort() method can sort the list in place and permanently alter the original order of the list. Whereas sorted() creates a new list, leaving the original list unchanged.


This article briefly discussed merge sort in python. Merge Sort is a sorting algorithm that uses the divide and conquers approach by repeatedly dividing an array into smaller sub-arrays and then merging them back in sorted order. It has a time complexity of O(n log n), which is an efficient and stable sort algorithm.

We hope this blog has helped you learn about merge sort in python. If you like to learn more, you can check out our articles: 

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