Java Program for QuickSort

13 Jun, 2022
Like Merge Sort, QuickSort is a Divide and Conquer algorithm. It picks an element as pivot and partitions the given array around the picked pivot. There are many different versions of quickSort that pick pivot in different ways.
Always pick first element as pivot.Always pick last element as pivot (implemented below)Pick a random element as pivot.Pick median as pivot.
Always pick first element as pivot.
Always pick last element as pivot (implemented below)
Pick a random element as pivot.
Pick median as pivot.
The key process in quickSort is partition(). Target of partitions is, given an array and an element x of array as pivot, put x at its correct position in sorted array and put all smaller elements (smaller than x) before x, and put all greater elements (greater than x) after x. All this should be done in linear time. Pseudo Code for recursive QuickSort function :
/* low  --> Starting index,  high  --> Ending index */
quickSort(arr[], low, high)
{
    if (low < high)
    {
        /* pi is partitioning index, arr[p] is now
           at right place */
        pi = partition(arr, low, high);

        quickSort(arr, low, pi - 1);  // Before pi
        quickSort(arr, pi + 1, high); // After pi
    }
}
Java
// Java program for implementation of QuickSortclass QuickSort{    /* This function takes last element as pivot,       places the pivot element at its correct       position in sorted array, and places all       smaller (smaller than pivot) to left of       pivot and all greater elements to right       of pivot */    int partition(int arr[], int low, int high)    {        int pivot = arr[high];        int i = (low-1); // index of smaller element        for (int j=low; j<high; j++)        {            // If current element is smaller than or            // equal to pivot            if (arr[j] <= pivot)            {                i++;                 // swap arr[i] and arr[j]                int temp = arr[i];                arr[i] = arr[j];                arr[j] = temp;            }        }         // swap arr[i+1] and arr[high] (or pivot)        int temp = arr[i+1];        arr[i+1] = arr[high];        arr[high] = temp;         return i+1;    }      /* The main function that implements QuickSort()      arr[] --> Array to be sorted,      low  --> Starting index,      high  --> Ending index */    void sort(int arr[], int low, int high)    {        if (low < high)        {            /* pi is partitioning index, arr[pi] is              now at right place */            int pi = partition(arr, low, high);             // Recursively sort elements before            // partition and after partition            sort(arr, low, pi-1);            sort(arr, pi+1, high);        }    }     /* A utility function to print array of size n */    static void printArray(int arr[])    {        int n = arr.length;        for (int i=0; i<n; ++i)            System.out.print(arr[i]+" ");        System.out.println();    }     // Driver program    public static void main(String args[])    {        int arr[] = {10, 7, 8, 9, 1, 5};        int n = arr.length;         QuickSort ob = new QuickSort();        ob.sort(arr, 0, n-1);         System.out.println("sorted array");        printArray(arr);    }}/*This code is contributed by Rajat Mishra */
Time Complexity: Worst case time complexity is O(N2) and average case time complexity is O(N logN)
Auxiliary Space: O(1)
Please refer complete article on QuickSort for more details!
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