randomized quicksort

#include <vector>
#include <iostream>
#include <cstdlib>
#include <ctime>

using namespace std;

void QuickSort(vector<int> &a, int l, int r);
int partiziona(vector<int> &a, int l, int r);
void scambia(int &a, int &b);

int main()
{
    srand((unsigned)time(0));
    cout<<"Inserisci il numero di elementi da ordinare del vettore: ";
    int tot;
    cin>>tot;
    vector<int> arr1;
    int x;
    cout<<"Inserimento: "<<endl;
    for(int i=0; i < tot ; i++){
        cin>>x;
        arr1.push_back(x);
    }
    cout<<"\nQuickSort"<<endl;
    QuickSort(arr1,0,tot-1); //tot-1 = arr1.size()-1;
    for(auto z : arr1)
        cout<<"\n"<<z;
    return 0;
}

void QuickSort(vector<int> &a, int l,  int r){
if(l >= r)return;
    int m = partiziona(a,l,r);
    QuickSort(a,l,m-1);
    QuickSort(a,m+1,r);
}

int partiziona(vector<int> &a, int l, int r)
{
    int random_ind = l+rand()%(r-l+1);
    int pivot = a.at(random_ind), J = l, i;
    for(i = l; i < r; i++)
        if(pivot >= a.at(i))
            scambia(a.at(i), a.at(J++));
    scambia(a.at(i), a.at(J));
    return J;
}
// | 1 | | 3 | | 2 |
void scambia(int &a, int &b)
{
    int temp;
    temp = a;
    a = b;
    b = temp;
}