WTF is Merge Sort?

In the world of computer science, sorting is one of the most fundamental problems. It's about arranging items in a certain order, typically numerical or lexicographical. While this might sound simple, the efficiency of the sorting algorithm can significantly impact the performance of complex systems, from databases to search engines. Among the plethora of sorting algorithms, Merge Sort stands out for its efficiency and simplicity. So, WTF is Merge Sort? Let's dive in.

The Basics of Merge Sort

Merge Sort is a "divide and conquer" algorithm. It divides the input array into two halves, calls itself for the two halves, and then merges the two sorted halves. The main principle behind the algorithm is that it's easier to merge two sorted arrays into one sorted array than to sort an array in one go.

How Does It Work?

The magic of Merge Sort is best explained through its implementation. Here's a Java snippet that encapsulates the essence of Merge Sort:

public static void divide(int[] arr, int si, int ei){
    if(si >= ei){
    int mid = si + (ei - si) / 2;
    divide(arr, si, mid);
    divide(arr, mid + 1, ei);
    conquer(arr, si, mid, ei);

public static void conquer(int[] arr, int si, int mid, int ei){
    int[] merged = new int[ei - si + 1];
    int idx1 = si;
    int idx2 = mid + 1;
    int x = 0;

    while(idx1 <= mid && idx2 <= ei){
        if(arr[idx1] <= arr[idx2]){
            merged[x++] = arr[idx1++];
        } else {
            merged[x++] = arr[idx2++];

    while(idx1 <= mid){
        merged[x++] = arr[idx1++];

    while(idx2 <= ei){
        merged[x++] = arr[idx2++];

    for(int i = 0, j = si; i < merged.length; i++, j++){
        arr[j] = merged[i];

The divide method recursively splits the array until it's broken down into single elements. The conquer method then takes over, merging these elements in a sorted manner. This process of splitting and merging continues until the entire array is sorted.

Fun Facts from History

Merge Sort was invented by John von Neumann in 1945, making it one of the earliest sorting algorithms. Von Neumann was a Hungarian-American mathematician, physicist, computer scientist, and polymath. He made major contributions to a number of fields, including mathematics, physics (particularly quantum mechanics), economics, computer science, and statistics.

One fun fact about Merge Sort is that it was developed during the era of early computing. At that time, computers were vastly different from what we use today, and efficient algorithms like Merge Sort were crucial for performing computations within a reasonable timeframe.

Why Merge Sort?

Merge Sort is beloved by programmers for several reasons:

  • Stable Sorting: Merge Sort does not change the relative order of elements with equal keys. This is an important property for certain applications.

  • Efficiency: It has a time complexity of O(n log n) in the worst case, which is as good as it gets for comparison-based sorting.

  • Parallelizable: The divide-and-conquer nature of Merge Sort makes it suitable for parallel computing.


Merge Sort is a classic algorithm that exemplifies the power of the divide-and-conquer strategy. Its development by John von Neumann highlights its historical significance and its role in the evolution of computing. Understanding how Merge Sort works is not just about learning to sort an array; it's about appreciating the elegance and efficiency of algorithmic problem-solving. So, the next time you hear about Merge Sort, you'll know exactly WTF it is and why it matters.