Data Science

What is the Time Complexity of Merge Sort Algorithm?

Published: 12th June, 2024

Arunav Goswami

Data Science Consultant at almaBetter

Learn about the merge sort time complexity, an efficient sorting algorithm. Discover its best, average, and worst-case scenarios and practical applications

Merge sort is one of the most efficient sorting algorithms available, widely used due to its reliable performance and time complexity characteristics. Understanding the time complexity of merge sort helps in analyzing its efficiency compared to other sorting algorithms like quicksort or bubble sort. In this article, we will delve into the time complexity of Merge Sort, breaking it down step-by-step to understand how it operates and why it is considered efficient.

Introduction to Merge Sort

Merge sort is a comparison-based sorting algorithm that follows the divide-and-conquer paradigm. Developed by John von Neumann in 1945, merge sort works by recursively dividing the unsorted list into smaller sublists until each sublist contains a single element. Then, it merges these sublists to produce a sorted list. This process is repeated recursively until the entire array is sorted. Merge Sort has an average and worst-case time complexity of O(n log n), making it a reliable choice for sorting large datasets. This systematic approach ensures that merge sort is both stable and has predictable performance characteristics.

The Divide-and-Conquer Approach

Dividing the Array

The first step in Merge Sort is dividing the array into two roughly equal halves. This division continues recursively until each sub-array contains a single element. The division process itself takes O(log n) time because each level of recursion halves the size of the array.

Sorting and Merging

Once the array is divided into individual elements, the merging process begins. During merging, two sorted sub-arrays are combined into a single sorted array. This process involves comparing elements from each sub-array and placing the smaller element into the final sorted array. The merging process for each pair of sub-arrays takes linear time, O(n).

The Merge Sort Process

The Merge Sort Process

Merge Sort Algorithm Time Complexity

In Merge Sort, the best, average, and worst-case time complexities are all O(n log n). This consistency is due to the algorithm always dividing the array into two halves and merging them, regardless of the initial order of elements.

Best Case Time Complexity of Merge Sort

The best case for merge sort occurs when the array is already sorted. However, unlike other algorithms such as quicksort, merge sort’s time complexity does not improve with a sorted array because it still needs to divide and merge the array completely.

Best Case Time Complexity:


Despite the array being sorted, merge sort still performs the division and merging steps, which means the best case time complexity remains O(nlogn).

What is the Average Case Time Complexity of Merge Sort

The average case time complexity of merge sort reflects its performance for a random array. Since merge sort consistently divides the array and processes each element in a systematic manner, the average case is similar to the best case.

Average Case Time Complexity:


This makes merge sort a reliable option for sorting, as its performance does not degrade significantly with different input data patterns.

Worst Case Time Complexity of Merge Sort

The worst case for merge sort occurs when the array is in reverse order or any other disordered state. Even in these scenarios, merge sort maintains its efficiency because it follows the same divide-and-conquer steps.

Worst Case Time Complexity:


Since merge sort consistently splits and merges subarrays, its worst-case time complexity is also O(nlogn), making it a robust choice for sorting large datasets.

Analyzing the Time Complexity of Merge Sort

To understand why merge sort has a time complexity of O(n log n), we need to look at the recurrence relation that defines its behavior:

T(n) = 2T(n/2) + O(n)

This relation represents the two recursive calls on subarrays of size n/2 and the linear time O(n) needed to merge the sorted subarrays.

Using the master theorem for divide-and-conquer recurrences:

T(n) = aT(n/b) + f(n)

where a = 2, b = 2, and f(n) = O(n)

Here, log_b a = log_2 2 = 1, and since f(n) = O(n), it fits the case f(n) = O(n log_b a ⋅ log^k n) for k = 0.

Therefore, by the master theorem:

T(n) = O(n log n)

This confirms that the time complexity of merge sort, in the best, average, and worst cases, is O(n log n).

Space Complexity of Merge Sort

In addition to time complexity, understanding the space complexity of merge sort is crucial. Merge Sort requires additional space proportional to the size of the array being sorted, leading to a space complexity of O(n). This is because it uses a temporary array to hold elements during the merge process.

Space Complexity:

S(n) = O(n)

Comparison with Other Sorting Algorithms

Quick Sort

Quick Sort, another popular sorting algorithm, has an average-case time complexity of O(n log n) but can degrade to O(n^2) in the worst case. Unlike Merge Sort, Quick Sort is an in-place sorting algorithm, which means it requires less additional space.

Bubble Sort and Insertion Sort

Both Bubble Sort and Insertion Sort have an average and worst-case time complexity of O(n^2), making them less efficient than Merge Sort for large datasets. These algorithms are generally more suitable for small or nearly sorted arrays.

Heap Sort

Heap Sort also has a time complexity of O(n log n) but differs in that it uses a binary heap data structure to sort elements. While it is in-place like Quick Sort, it is not a stable sort, meaning the relative order of equal elements may not be preserved.

Practical Applications of Merge Sort

Merge Sort is particularly useful in scenarios where stability is important, such as:

Sorting Linked Lists

  • Linked lists benefit from Merge Sort's non-reliance on random access, making it more efficient than Quick Sort for this data structure.

External Sorting

  • When sorting large datasets that do not fit into memory, Merge Sort is used in external sorting algorithms because it can handle large amounts of data by dividing it into manageable chunks.

Code Implementation

Here's a simple implementation of Merge Sort in Python:



Merge Sort is a highly efficient and stable sorting algorithm with a consistent time complexity of O(n log n). Its divide-and-conquer approach and its reliable performance across best, average, and worst-case scenarios make it a preferred choice for sorting large datasets. While it requires additional space, its stability and efficiency often outweigh this drawback, especially in applications like sorting linked lists and external sorting. By understanding the time complexity and practical applications of Merge Sort, developers can make informed decisions about when to use this algorithm for optimal performance.

If you found this article helpful and are eager to delve further into algorithms and data structures, explore our in-depth courses: Data Science Training, Web Development Course, and Masters in Data Science. Boost your skills and master the latest technologies and methodologies in data science and web development.

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