Why Randomized Quicksort is Preferred Over Merge Sort

Comparative Analysis of Randomized Quicksort and Mergesort in In-built Libraries: A Dive into Performance, Implementation, and Application

Randomized quicksort and mergesort are both powerful sorting algorithms utilized in various programming environments. However, many in-built modules and libraries often favour randomized quicksort over mergesort. This preference is rooted in several technical and practical considerations that make randomized quicksort a more attractive choice in many scenarios. This article delves into these considerations and provides examples to elucidate the rationale behind this preference.

First, look into both algorithms

Randomized Quicksort

import java.util.Random;

public class RandomizedQuickSort {

    public static void quickSort(int[] arr, int low, int high) {
        if (low < high) {
            int pivotIndex = randomizedPartition(arr, low, high);
            quickSort(arr, low, pivotIndex - 1);
            quickSort(arr, pivotIndex + 1, high);

    private static int randomizedPartition(int[] arr, int low, int high) {
        int randomIndex = new Random().nextInt(high - low + 1) + low;
        swap(arr, randomIndex, high);  // Swap random element with the last element
        return partition(arr, low, high);

    private static int partition(int[] arr, int low, int high) {
        int pivot = arr[high];
        int i = low - 1;
        for (int j = low; j < high; j++) {
            if (arr[j] <= pivot) {
                swap(arr, i, j);
        swap(arr, i + 1, high);
        return i + 1;

    private static void swap(int[] arr, int i, int j) {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;

    public static void main(String[] args) {
        int[] arr = {10, 7, 8, 9, 1, 5};
        quickSort(arr, 0, arr.length - 1);
        for (int i : arr) {
            System.out.print(i + " ");

For Randomized Quicksort:

  • The quickSort method is the entry point to the algorithm.
  • The randomizedPartition method selects a random pivot to help partition the array.
  • The partition method organizes the elements such that all elements to the left of the pivot are lesser, and to the right are greater.
  • The swap method is a utility function to swap the elements in the array.
  • The main method demonstrates the usage of the algorithm with a sample array.

Merge Sort

public class MergeSort {

    public static void mergeSort(int[] arr, int left, int right) {
        if (left < right) {
            int mid = (left + right) / 2;
            mergeSort(arr, left, mid);
            mergeSort(arr, mid + 1, right);
            merge(arr, left, mid, right);

    private static void merge(int[] arr, int left, int mid, int right) {
        int n1 = mid - left + 1;
        int n2 = right - mid;
        int[] leftArr = new int[n1];
        int[] rightArr = new int[n2];

        System.arraycopy(arr, left, leftArr, 0, n1);
        System.arraycopy(arr, mid + 1, rightArr, 0, n2);

        int i = 0, j = 0;
        int k = left;
        while (i < n1 && j < n2) {
            if (leftArr[i] <= rightArr[j]) {
                arr[k++] = leftArr[i++];
            } else {
                arr[k++] = rightArr[j++];

        while (i < n1) {
            arr[k++] = leftArr[i++];

        while (j < n2) {
            arr[k++] = rightArr[j++];

    public static void main(String[] args) {
        int[] arr = {12, 11, 13, 5, 6, 7};
        mergeSort(arr, 0, arr.length - 1);
        for (int i : arr) {
            System.out.print(i + " ");

For Mergesort:

  • The mergeSort method recursively splits and sorts the array.
  • The merge method merges two sorted sub-arrays into a single sorted array.
  • The main method demonstrates the usage of the algorithm with a sample array.

Algorithm Complexity

Both randomized quicksort and mergesort boast an average time complexity of O(n log n), which signifies their efficiency. However, their behaviour differs in practice due to constant factors and real-world scenarios.

  • Randomized Quicksort: Randomized Quicksort tends to have smaller constant factors, which makes it faster in practice for many input distributions. The randomization helps in avoiding the worst-case scenario of O(n^2), which can occur in deterministic quicksort.
  • Mergesort: Mergesort, on the other hand, maintains a consistent performance of O(n log n), but its constant factors are usually larger due to the additional memory allocation and copying steps involved.

In-Place Sorting

One of the compelling advantages of randomized quicksort is its in-place sorting capability.

  • Example: Consider an array of integers. Randomized quicksort can sort this array using only a small amount of additional memory, which is a significant advantage in memory-constrained environments.

Mergesort, conversely, requires additional memory equal to the size of the input array, which can be prohibitive in memory-sensitive scenarios.

Cache Performance

The cache performance of an algorithm can significantly impact its real-world performance. Randomized quicksort often exhibits better cache performance due to its in-place nature and sequential access pattern.

  • Example: In a system with a hierarchical memory structure, the in-place operation of randomized quicksort ensures that most of the data stays in the faster levels of memory (like the cache), which can significantly speed up the sorting process.

Parallelism and Scalability

Both algorithms have potential for parallelization, but their scalability differs.

  • Randomized Quicksort: Its divide-and-conquer strategy lends itself well to parallelization. The partitioning step can be performed in parallel on multiple sections of the array, which can lead to a substantial reduction in sorting time.
  • Mergesort: While mergesort is also amenable to parallelization, the merging step can become a bottleneck, especially when dealing with large datasets.

Implementation Simplicity

Randomized quicksort is often perceived as simpler to implement and understand, which can be a factor in its preference.

  • Example: The core of randomized quicksort can be implemented with just a few lines of code, making it an attractive choice for in-built libraries aiming for minimalism and efficiency.


While mergesort is a stable sorting algorithm (i.e., it maintains the relative order of equal elements), randomized quicksort is not. However, in many practical applications, stability is not a crucial requirement, and the other advantages of randomized quicksort outweigh this aspect.


The preference for randomized quicksort in many in-built modules and libraries is grounded in its in-place sorting capability, better cache performance, simplicity of implementation, and often superior real-world performance. While mergesort has its own set of advantages, particularly its stability and predictable performance, the trade-offs often favour randomized quicksort, making it a more common choice in the realm of sorting algorithms within in-built modules.

Keep exploring, keep learning, and keep coding!

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