TheAlgorithms-C-Plus-Plus/sorting/quick_sort.cpp

/**
 *
 *      copyright The Algorithms
 *      Author - 
 *      Correction - ayaankhan98
 *
 * Implementation Details -
 *      Quick Sort is a divide and conquer algorithm. It picks and element as
 *      pivot and partition the given array around the picked pivot. There
 *      are many different versions of quickSort that pick pivot in different
 *      ways.
 *
 *      1. Always pick the first element as pivot
 *      2. Always pick the last element as pivot (implemented below)
 *      3. Pick a random element as pivot
 *      4. Pick median as pivot
 *
 *      The key process in quickSort is partition(). Target of partition is,
 *      given an array and an element x(say) of array as pivot, put x at it's
 *      correct position in sorted array and put all smaller elements (samller
 *      than x) before x, and put all greater elements (greater than x) after
 *      x. All this should be done in linear time
 *
 */

#include <cstdlib>
#include <iostream>

/* 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];    // taking the last element as pivot
    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++;   // increment index of smaller element
            int temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        }
    }
    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 quickSort(int arr[], int low, int high) {
    if (low < high) {
        int p = partition(arr, low, high);
        quickSort(arr, low, p - 1);
        quickSort(arr, p + 1, high);
    }
}

// prints the array after sorting
void show(int arr[], int size) {
    for (int i = 0; i < size; i++)
        std::cout << arr[i] << " ";
    std::cout << "\n";
}

// Driver program to test above functions
int main() {
    int size;
    std::cout << "\nEnter the number of elements : ";
    std::cin >> size;
    int *arr = new int[size];
    std::cout << "\nEnter the unsorted elements : ";
    for (int i = 0; i < size; ++i) {
      std::cin >> arr[i];
    }
    quickSort(arr, 0, size-1);
    std::cout << "Sorted array : ";
    show(arr, size);
    return 0;
}