Finding Quartils with cpp - c++

Question

Problem:
Find the 3 quartils of a set;
The first line contains an integer, n , denoting the number of elements in the array.
The second line contains n space-separated integers describing the array's elements.
So what are quartils? There are 3 quartiles, the 2nd one is the median, the middle element of your set, if n%2==0 is the sum of the middle 2 devided by 2, the 1st and the 3rd one, we get the first and the second half of our set and we find the medians of these sets, that basicly all, here is full info( https://en.wikipedia.org/wiki/Quartile ).
So I want to submit my code in hackerrank and I can't seem to find my mistake, the test case passes, I made my test and everything is working find but I can't seem to find my mistake, can you help me find it but generally.. what is the way to debug this type of problem where everything is working but the output is not fine for some output ? Can unit testing help me out here? Here is my code:
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
// A utility function to swap two elements
void swap(int* a, int* b)
{
int t = *a;
*a = *b;
*b = t;
}
/* 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]; // pivot
int i = (low - 1); // Index of smaller element
for (int j = low; j <= high- 1; j++)
{
// If current element is smaller than or
// equal to pivot
if (arr[j] <= pivot)
{
i++; // increment index of smaller element
swap(&arr[i], &arr[j]);
}
}
swap(&arr[i + 1], &arr[high]);
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)
{
/* pi is partitioning index, arr[p] is now
at right place */
int pi = partition(arr, low, high);
// Separately sort elements before
// partition and after partition
quickSort(arr, low, pi - 1);
quickSort(arr, pi + 1, high);
}
}
int getQuartil(int arr[],int len){
int q=0;
if(len%2){
q=arr[len/2];
}else{
q=(arr[len/2]+arr[len/2-1])/2;
}
return q;
}
int main() {
int n;
std::cin >> n;
int arr[n];
for(int i =0; i < n; i++){
std::cin >> arr[i];
}
quickSort(arr,0,n);
int q1=getQuartil(arr,n/2);
int q2=getQuartil(arr, n);
int q3=getQuartil(arr, (n+n/2+1));
std::cout << q1 << std::endl;
std::cout << q2 << std::endl;
std::cout << q3 << std::endl;
return 0;
}