Merge Sort for Linked List: A Step-by-Step Guide

Sorting is an indispensable operation in the realm of algorithms and data structures. It enables us to efficiently organize and manipulate data. Merge Sort is a well-known sorting algorithm that is known for its efficacy and adaptability. This article will examine the concept of Merge Sort as it pertains to linked lists. Merge Sort for linked lists is essential knowledge whether you are a computer science student or a programmer seeking to optimize their code.

Overview of Merge Sort

In the context of Merge Sort for a linked list, “merged list” refers to the final sorted linked list produced by combining and rearranging the elements of two lesser sorted linked lists. 

Here is how the consolidated list is created during the Merge Sort procedure:

Splitting: Each of the two halves of the original unsorted linked list is recursively sorted. These halves are known as the “left half” and “right half.”

Sorting: Individually sorting the left and right halves of the linked list using the Merge Sort algorithm. This indicates that the elements within each half have been rearranged in ascending sequence.

Merging: After sorting both halves, they are recombined into a single sorted linked list. This is accomplished by comparing each element in the left and right halves and arranging them in ascending order within the merged list.

Result: The final output of the Merge Sort algorithm is the merged list, which comprises all of the elements of the original linked list in sorted order.

The merged list is the final output of the Merge Sort algorithm applied to a linked list. It is a unique linked list containing each element of the input list in ascending order.

Why Merge Sort for Linked Lists?

Merge Sort for linked lists is frequently selected for linked lists due to its numerous benefits for this data structure.

Stability: Bring together Sort is a stable sorting algorithm, which means it maintains the relative order of elements of equal value. This property is essential in the context of linked lists if you wish to preserve the original order of elements with the same key.

Efficiency: Merge Sort has a constant time complexity of O(n log n), even for linked collections. This makes it effective for large datasets, and unlike some other sorting algorithms, its performance is not affected by the initial order of elements.

No Requirement for Random Access: Unlike arrays, linked lists do not provide arbitrary access to elements in constant time. Merge Sort is appropriate for linked lists because it does not rely on random access. It requires only sequential access, which is a characteristic of linked lists.

Adaptability: Bring together Sort can be adapted without significant modifications to sort linked lists. The stages of divide, sort, and merge can be implemented recursively using the structure of linked lists.

No Additional Space (In-Place Variant): While the classic Merge Sort algorithm typically requires additional memory allocation for merging, there is an in-place variant called “Bottom-Up Merge Sort” that can be used with linked lists and does not require additional memory allocation. This can be beneficial in limited memory situations.

Predictable Performance: Regardless of the input data, Merge Sort provides a predictable and consistent performance. This can be advantageous for real-time or safety-critical applications where performance consistency is essential.

Parallelism: Bring together Sort can be effectively parallelized, particularly when categorizing large datasets. Thus, it is compatible with multicore processors and distributed systems.

Merge Sort is a stable and efficient algorithm for sorting linked lists, offering benefits such as stability, efficiency, adaptability to the linked list data structure, and the ability to conduct in-place sorting when necessary. These qualities make it a popular sorting algorithm for linked lists.

How Merge Sort Works

Merge Sort is a divide-and-conquer sorting algorithm that works by dividing a list into smaller sublists, classifying those sublists, and then recombining them to generate a sorted list. Here is a detailed explanation of how Merge Sort for linked lists operates:

Split: Merge Sort begins by dividing the unsorted list into smaller sublists. This is repeated recursively until each sublist either contains one element or is vacant (at which point it is considered sorted). This procedure is repeated until there are numerous tiny sublists.

Conquer (of a Kind): After dividing the list into smaller sublists, the items are sorted. This is typically accomplished by comparing each sublist’s elements and rearranging them in ascending order. The procedure of sifting continues until all sublists have been sorted.

Combine: After classification, the smaller sublists are combined to form larger sublists. These broader sublists are categorized during the process of merging. The merging is accomplished by comparing the elements of the sublists and arranging them in the correct order to produce a larger, sorted sublist.

Repetition: The first three steps are repeated recursively until there is only one sublist remaining, which is the sorted list.

Final Outcome: The final output is a thoroughly sorted list containing every constituent from the initial unsorted list.

Here is a high-level representation of the Merge Sort algorithm in pseudocode:

    if length of arr <= 1:
        return arr  // Base case: already sorted or empty

    // Divide the array into two halves
    mid = length of arr / 2
    left_half = arr[0:mid]
    right_half = arr[mid:]

    // Recursively sort both halves
    left_sorted = MergeSort(left_half)
    right_sorted = MergeSort(right_half)

    // Merge the sorted halves
    sorted_arr = Merge(left_sorted, right_sorted)

    return sorted_arr

By comparing and rearranging elements, the ‘Merge’ function merges two sorted subarrays into a single sorted array. The recursion continues until the array is completely sorted.

Merge Sort is renowned for its stable O(n log n) time complexity, which makes it suitable for processing large datasets, and its ability to preserve relative element order (stability). It is frequently employed when a stable, consistent algorithm for categorizing is required.

Pseudocode for Merge Sort

Here’s a simplified pseudocode representation of the Merge Sort algorithm for linked lists:

    if head is None or is None:
        return head

    # Split the linked list into two halves
    middle = findMiddle(head)
    left = head
    right = = None

    # Recursively sort and merge the two halves
    left = MergeSort(left)
    right = MergeSort(right)

    # Merge the sorted halves
    sortedList = merge(left, right)
    return sortedList

## 5. Implementing Merge Sort for Linked Lists in Python

Let’s implement Merge Sort for linked lists in Python:

# Python code for Merge Sort on Linked List

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val = next

def mergeSort(head):
    if head is None or is None:
        return head

    # Find the middle of the linked list
    middle = findMiddle(head)
    left = head
    right = = None

    # Recursively sort and merge the two halves
    left = mergeSort(left)
    right = mergeSort(right)

    # Merge the sorted halves
    sortedList = merge(left, right)
    return sortedList

Analysis of Time and Space Complexity

Time Complexity

Merge Sort has a time complexity of O(n log n), making it an extremely efficient sorting algorithm for large datasets.

Space complexity

Merge Sort has a space complexity of O(n) owing to the additional memory required for merging.

Advantages of Merge Sort for Linked Lists

Merge Sort offers several advantages when applied to linked lists:

Stability: Merge Sort is a stable sorting algorithm. It preserves the relative order of equal elements. In the context of linked lists, this means that if you have nodes with the same value, their original order will be maintained in the sorted result.

Consistent Performance: Merge Sort has a consistent time complexity of O(n log n) for sorting linked lists, regardless of the initial order of elements. This consistent performance makes it a reliable choice for sorting linked lists of varying sizes.

Efficient Memory Usage: Merge Sort for linked lists doesn’t require additional memory space proportional to the size of the list. It is a memory-efficient sorting algorithm, which can be important when dealing with limited memory resources.

No Random Access Required: Linked lists do not provide constant-time random access to elements, making algorithms like Quick Sort less efficient. Merge Sort, on the other hand, only requires sequential access to the elements, making it well-suited for linked lists.

Adaptability: Merge Sort can be adapted to work with linked lists with relative ease. The recursive divide-and-conquer approach aligns well with the structure of linked lists.

Predictable Performance: Merge Sort provides consistent and predictable performance. It doesn’t exhibit worst-case scenarios like some other sorting algorithms, making it a safe choice for real-time or critical applications.

Parallelization: Merge Sort can be parallelized effectively, especially for sorting large linked lists. This allows you to take advantage of multi-core processors and distributed systems for faster sorting.

External Sorting: Merge Sort is commonly used for external sorting, where data is too large to fit into memory. It can efficiently handle large datasets by dividing them into smaller chunks, sorting them in memory, and then merging them back together.

Ease of Implementation: The merge operation in Merge Sort is straightforward to implement for linked lists, making it a suitable choice for developers who need to sort linked data structures.

Merge Sort offers stability, consistent performance, memory efficiency, adaptability to linked lists, and other advantages that make it a popular choice for sorting linked lists in various applications.

Merge Sort is compared to other sorting algorithms.

Merge Sort versus Quick Sort

Quick Sort is a second efficient sorting algorithm, but it is unstable and may perform inadequately with certain datasets.

Merge Sort versus Bubble Sort

Bubble Sort is a straightforward but inefficient algorithm for categorizing large datasets. In most situations, Merge Sort outperforms Bubble Sort.

Real-World Implementations

Merge Sort is extensively utilized in a variety of applications, such as: –

External categorizing in databases

Merge operations in merge files-classify file systems

Algorithms for collaborative filtration in recommendation systems


Merge Sort is a versatile and effective sorting algorithm for linked lists. It is a valuable instrument in computer science and software development due to its adaptability to linked lists. Merge Sort can substantially enhance the efficacy of your code. Now that you have an in-depth comprehension of Merge Sort for linked lists, you can confidently implement this sorting algorithm in your programming projects.


Can Merge Sort be used for arrays as well?

Yes, Merge Sort can be used for both linked lists and arrays, but it’s particularly efficient for linked lists.

Is Merge Sort a stable sorting algorithm?

Yes, Merge Sort is a stable sorting algorithm, meaning it preserves the relative order of equal elements.

What is the worst-case time complexity of Merge Sort?

The worst-case time complexity of Merge Sort is O(n log n).

Are there in-place variations of Merge Sort?

Yes, there are in-place variations of Merge Sort that reduce the space complexity to O(1), but they are more complex to implement.

How does Merge Sort compare to other sorting algorithms in terms of speed?

Merge Sort is generally faster than Bubble Sort and more efficient than Quick Sort for large datasets.

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