Python Tutorial



Python Tutorial

Merge Sort in Python

A fundamental sorting technique in Python called merge sort effectively manages huge datasets. In this thorough article, we'll go deep into merge sort, looking at its principles, Python implementation, time complexity, optimizations, and more. Let's start on the path to learning the skill of effective sorting. A well-liked and effective sorting algorithm in Python is called merge sort; it employs the divide-and-conquer strategy. This method entails breaking a problem down into numerous smaller issues. Then, each sub-problem is resolved on its own. Sub-problems are finally integrated to get the complete solution.

Introduction to Merge Sort

A traditional sorting algorithm based on the divide and conquer method is merge sort. The unsorted list is split into n sublists, each of which has one element, and each of these sublists is continuously merged to create a new sorted sublist until only one sublist is left. The sorted list is the last sublist. One of the most effective sorting algorithms is merge sort. Its foundation is the divide-and-conquer tactic. A list is constantly divided into several sublists using the merge sort algorithm until each sublist contains just one item.


At its core, merge sort operates by breaking down the problem into simpler subproblems, sorting them individually, and then merging the sorted sublists to obtain a fully sorted list. This approach ensures stability and predictable performance, making it a preferred choice in various applications. Merge sort is one of the most efficient sorting algorithms. It works on the principle of Divide and Conquer based on the idea of breaking down a list into several sub-lists until each sublist consists of a single element and merging those sublists in a manner that results in a sorted list.

Merge Sort Concept

How does Divide and Conquer Work?

Divide and conquer is a problem-solving strategy where a problem is divided into smaller, more manageable subproblems. In the case of merge sort, the unsorted list is divided into smaller sublists until each sublist contains only one element. These single-element sublists are then merged back together, ensuring that the merged list is sorted.

Let us have a rough understanding of merge sort:

  1. Consider an array

  2. Find the middle point to divide the array into two halves

  3. Call merge sort for the first half

  4. Call merge sort for the second half

  5. Merge both the half

  6. The result will be in a sorted format

Python Programs for Merge Sort

Let's dive into Python implementations of merge sort. Below is a step-by-step guide to sorting an array using the merge sort algorithm.


As of now, we have a rough understanding of how merge sort is performed. For better understanding, let's dive deep into the algorithm followed by the code:

  1. Create a merge_sort() function.

  2. Initiate array list and divide it into subarrays.

  3. Create copies of the subarrays.

  4. Create three-pointers that maintain indexes.

  5. Pick larger elements and place them in the right position.

  6. Pick the remaining elements and sort them accordingly.

  7. The result will be a sorted array.

  8. Print the sorted array.

Sorting Array

def merge_sort(arr):
    if len(arr) > 1:
        mid = len(arr) // 2  # Find the middle of the array
        left_half = arr[:mid]  # Divide the array into two halves
        right_half = arr[mid:]

        merge_sort(left_half)  # Recursive call on the left half
        merge_sort(right_half)  # Recursive call on the right half

        i = j = k = 0

        # Copy data to temporary lists left_half[] and right_half[]
        while i < len(left_half) and j < len(right_half):
            if left_half[i] < right_half[j]:
                arr[k] = left_half[i]
                i = 1
                arr[k] = right_half[j]
                j = 1
            k = 1

        # Check if any element was left
        while i < len(left_half):
            arr[k] = left_half[i]
            i = 1
            k = 1

        while j < len(right_half):
            arr[k] = right_half[j]
            j = 1
            k = 1

# Example usage
arr = [12, 11, 13, 5, 6, 7]
print("Sorted array is:", arr)

This Python program demonstrates merge sort on an array.

The Time Complexity of Merge Sort in Python

Merge sort exhibits a time complexity of O(nlogn) as it consistently divides the unsorted list into smaller sublists until each sublist contains only one element. The merging process takes linear time, resulting in a balanced and efficient performance across various input sizes.

  1. Optimal Performance: Merge sort's time complexity of O(nlogn) ensures optimal performance for both small and large datasets. This efficiency remains consistent, making it a reliable choice regardless of the input size.

  2. Predictable Behavior: Unlike some other sorting algorithms, merge sort exhibits a consistent performance regardless of the initial ordering of elements in the input list. Whether the list is partially sorted, completely unsorted, or almost sorted, merge sort guarantees the same O(nlogn) time complexity.

  3. Parallelization Potential: Merge sort's divide and conquer nature enables parallelization, a significant advantage in modern computing environments. Large datasets can be divided into smaller chunks and sorted simultaneously, leveraging the full processing power of multicore systems.

  4. Adaptability to Linked Lists: Merge sort is highly adaptable to linked lists, making it an ideal choice for sorting data structures where random access is costly, such as linked lists. Its ability to merge linked lists efficiently further underscores its versatility.

  5. Stability and Preservation of Order: Merge sort is a stable sorting algorithm that maintains the relative order of equal elements in the sorted output as they were in the input. This stability is crucial in various applications where maintaining the initial order of equivalent elements is essential.

  6. Space Complexity: While merge sort boasts an optimal time complexity, it does require additional space proportional to the input size due to the need for temporary arrays during the merging process. Despite this space overhead, its stability, efficiency, and adaptability often outweigh this minor drawback.


While merge sort inherently offers stable and predictable performance, certain optimizations can further enhance its efficiency. One such optimization involves using insertion sort for small sublists. Insertion sort performs efficiently for small datasets, making it an ideal choice for optimizing merge sort for smaller inputs. Merge sort is one of the most efficient sorting algorithms. It works on the principle of Divide and Conquer based on the idea of breaking down a list into several sub-lists until each sublist consists of a single element and merging those sublists in a manner that results in a sorted list. 


Implementing merge sort requires attention to detail. Ensuring the correct partitioning of the input list and the accurate merging of sublists are crucial steps in the implementation process. A precise implementation can be achieved by meticulously following the algorithm's steps. Merge Sort is a divide-and-conquer algorithm. It divides the input array into two halves, calls itself the two halves, and then merges the two sorted halves. The merge() function is used for merging two halves. 

  1. Divide and Conquer Strategy: Merge sort follows a clear divide and conquer strategy, where the unsorted list is divided into smaller sublists until each sublist contains only one element. This division simplifies the sorting process for individual elements, making merging them back into sorted sublists easier.

  2. Recursive Approach: One of the key features of merge sort is its recursive nature. The algorithm calls itself recursively to sort smaller sublists. This recursive approach ensures that the sorting process continues until the base case is reached (i.e., sublists with a single element), guaranteeing that every element is eventually sorted.

  3. Merging Sublists: The merging step is pivotal in merge sort. After breaking the list into single-element sublists, these sublists are merged back together in a sorted manner. The merging process involves comparing elements from the two sublists and arranging them in ascending or descending order, depending on the desired sorting order.

  4. Index Management: Proper management of indices is crucial during the merge step. Efficient merging requires keeping track of the current positions in both the left and right sublists. The algorithm ensures that the resulting list is sorted by comparing elements at these positions and merging them correctly. 

  5. In-Place Merge Sort: While merge sort typically uses additional memory for temporary arrays during merging, in-place merge sort is an advanced variant that avoids this extra space usage. It achieves in-place sorting by modifying the input list directly during the merging process, thus conserving memory at the cost of increased complexity in implementation.

  6. Stability and Adaptive Nature: Merge sort is stable, preserving the order of equal elements, and adaptive, meaning it performs well for partially sorted lists. These characteristics make it a preferred choice in applications where maintaining the initial order and adapting to various input scenarios are essential requirements.

  7. Parallelization Possibilities: The divide-and-conquer nature of merge sort allows for parallelization, enabling developers to leverage parallel processing architectures. Large datasets can be efficiently divided and sorted in parallel, harnessing the full computational power of modern multi-core processors.

Sorting Custom Objects

Merge sort can be extended to sort custom objects by defining a custom comparison function. This approach allows the algorithm to sort objects based on specific attributes, enabling diverse applications in real-world scenarios. Merge Sort divides an array into smaller subarrays, sorts them, and then merges them back together to achieve a sorted result. The code comprises two main functions: merge to combine two sorted arrays and mergesort to split and sort an array recursively. 


In conclusion, mastering merge sort in Python equips developers with a powerful tool for tackling sorting challenges with precision and efficiency. Its elegant divide-and-conquer approach, coupled with stable performance, makes it an indispensable algorithm in the world of computer science. By understanding the intricacies detailed in this guide, developers are well-prepared to optimize their code, ensuring seamless sorting experiences for even the most extensive datasets and complex custom objects. Embrace merge sort and empower your applications with a sorting solution that stands the test of time.


Q1: Is merge sort suitable for large datasets?

Yes, merge sort is well-suited for large datasets due to its efficient divide-and-conquer strategy, resulting in a time complexity of O(nlogn). Merge sort can work well on any type of data set irrespective of its size (either large or small). whereas The quick sort cannot work well with large datasets. Next, it says that merge sort is not in place because it requires additional memory space to store the auxiliary arrays. 

Q2: Can merge sort handle custom objects?

Certainly, merge sort can be extended to sort custom objects by defining a custom comparison function, allowing precise sorting based on specific object attributes. Sorting Array The merge sort algorithm divides the given array into roughly two halves and sorts them recursively. 

Q3: What is the primary advantage of merge sort over other sorting algorithms?

Merge sort's primary advantage lies in its stable and consistent O(nlogn) time complexity, ensuring reliable performance across various input sizes and data distributions. Merge sort can be used with linked lists without taking up any more space. A merge sort algorithm is used to count the number of inversions in the list. 

Q4. Why is merge sort used?

Use merge sort when there is a consideration for the stability of data. Stable sorting involves maintaining the order of identical values within an array. When compared with the unsorted data input, the order of identical values throughout an array in a stable sort is kept in the same position in the sorted output.

Q5. What is merge sort with two functions?

The Merge Sort algorithm operates through two functions: the mergeSort function itself that divides our input recursively and the merge function that sorts and stitches our divided halves back together into our sorted array output.

Leave a Reply

Your email address will not be published. Required fields are marked *