how to implement quick sort algorithm in C++

Question

here is the of quick sort algorithm from the MITOcw(Introduction To Algorithms ) lecture

QUICKSORT(A,p,q)
if(p < q)
then r = PARTITION(A,p,q)
     QUICKSORT         


        

Answer 1

Your consideration is wrong. The value of r does not change, since it is given as value to the Quicksort function(not a reference). You handle the ranges with p,q such that p is the first index in the range and q the first index not in the range.

Thus, your calls were wrong:

r=partition(A, p,q);
quickSort(A,p,r); //range is from A[p] to A[r-1] 
quickSort(A,(r+1),q); //range is from A[r+1] to A[q-1]

Here is the complete example. I used std::swap to change elements and ans std::vector instead of an array.

#include 
#include 

using namespace std;

void quickSort(vector&,int,int);

int partition(vector&, int,int);

int main()
{
    vector A = {6,10,13,5,8,3,2,25,4,11};
    int p=0;
    int q=10;

    cout<<"======Original======="<& A, int p,int q)
{
    int r;
    if(p& A, int p,int q)
{
    int x= A[p];
    int i=p;
    int j;

    for(j=p+1; j

Live example: ideone