Quick Sort in C++ with Algorithm, Example

Algorithm:

quickSort(array<T>& a)
{
    quickSort(a, 0, a.length);
    quickSort(array<T> & a, int i, int n)
    {
        if (n <= 1)
            return;
        T pi = a[i + rand() % n];
        int p = i - 1, j = i, q = i + n;
        while (j < q) {
            int comp = compare(a[j], pi);
            if (comp < 0) {
                a.swap(j++, ++p); // move to beginning of array
            }
            else if (comp > 0) {
                a.swap(j, --q); // move to end of array
            }
            else {
                j++; // keep in the middle
            }
        }
    }
}
// a[i..p]<pi, a[p+1..q-1]=pi, a[q..i+n-1]>pi
quickSort(a, i, p - i + 1);
quickSort(a, q, n - (q - i));

Consider the Quick sort working flow:

C++ program for Quick Sort

#include <iostream>
using namespace std;

void quicksort(int, int, int);
int partition(int, int, int);

int partition(int* a, int s, int e)
{
    int piviot = a[e];
    int pind = s;
    int i, t;

    for (i = s; i < e; i++) {
        if (a[i] <= piviot) {
            t = a[i];
            a[i] = a[pind];
            a[pind] = t;
            pind++;
        }
    }

    t = a[e];
    a[e] = a[pind];
    a[pind] = t;

    return pind;
}

void quicksort(int* a, int s, int e)
{
    if (s < e) {
        int pind = partition(a, s, e);
        quicksort(a, s, pind - 1);
        quicksort(a, pind + 1, e);
    }
}

int main()
{
    int n;

    cout << "Enter number of elements : " ;
    cin >> n;

    int a[n];

    cout << "Enter the array : ";
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }

    quicksort(a, 0, n - 1);

    cout << "Sorted array is : ";
    for (int i = 0; i < n; i++) {
        cout << a[i] << " ";
    }

    return 0;
}

Output

RUN 1:
Enter number of elements : 5
Enter the array : 10 20 5 15 10
Sorted array is : 5 10 10 15 20

RUN 2:
Enter number of elements : 10
Enter the array : 40 30 20 50 60 70 80 90 10 11
Sorted array is : 10 11 20 30 40 50 60 70 80 90