Quick Sorting in C++ is working intermittently - c++

Question

What is wrong with my quick sorting algorithm? Sometimes it gives the right answer, sometimes it doesn't. Here's my code.
#include <iomanip>
#include <iostream>
using namespace std;
void foo(int*, int, int);
int main()
{
int* a, n;
cin >> n;
a = new int[n];
srand(time(NULL));
for (int i = 0; i <= n - 1; i++)
{
a[i] = rand() % 100;
cout << setw(3) << a[i];
}
cout << endl;
cout << "sorting array" << endl;
foo(a, 0, n - 1);
for (int i = 0; i <= n - 1; i++)
cout << setw(3) << a[i];
cout << endl;
return 0;
}
void foo(int* a, int left, int right)
{
int j = right;
int i = left;
int mid = (i + j) / 2;
while (i < j)
{
while (a[i] < a[mid]) i++;
while (a[j] > a[mid]) j--;
if (i <= j)
{
swap(a[i], a[j]); i++; j--;
}
}
if (i < right)
foo(a, i, right);
if (left < j) foo(a, left, j);
}
I've done some changed to my program And it started working properly, the problem is that I don't understand why. I mentioned the changed in the code below. All the expected outputs matches the result.
#include <iomanip>
#include <iostream>
using namespace std;
void foo(int*, int, int);
int main()
{
int* a, n;
cin >> n;
a = new int[n];
srand(time(NULL));
for (int i = 0; i <= n - 1; i++)
{
a[i] = rand() % 100;
cout << setw(3) << a[i];
}
cout << endl;
cout << "sorting array" << endl;
foo(a, 0, n - 1);
for (int i = 0; i <= n - 1; i++)
cout << setw(3) << a[i];
cout << endl;
return 0;
}
void foo(int* a, int left, int right)
{
int j = right;
int i = left;
int mid = (a[right] + a[left]) / 2;//CHANGED LINE from mid=(i+j)/2
while (i < j)
{
while (a[i] < mid) i++; //CHANGED LINE from a[i]<a[mid]
while (a[j] > mid) j--; //CHANGED LINE from a[j]>a[mid]
if (i <= j)
{
swap(a[i], a[j]); i++; j--;
}
}
if (i < right)
foo(a, i, right);
if (left < j) foo(a, left, j);
}