How to find the kth smallest number in cinstant space

#include <bits/stdc++.h> 
using namespace std; 
  
// Function to return the kth smallest 
// element from the array 
int kthSmallest(int* arr, int k, int n) 
{ 
  
    // Minimum and maximum element from the array 
    int low = *min_element(arr, arr + n); 
    int high = *max_element(arr, arr + n); 
  
    // Modified binary search 
    while (low <= high) { 
  
        int mid = low + (high - low) / 2; 
  
        // To store the count of elements from the array 
        // which are less than mid and 
        // the elements which are equal to mid 
        int countless = 0, countequal = 0; 
        for (int i = 0; i < n; ++i) { 
            if (arr[i] < mid) 
                ++countless; 
            else if (arr[i] == mid) // Equal elements were the real trouble
                ++countequal; 
        } 
  
        // If mid is the kth smallest 
        if (countless < k 
            && (countless + countequal) >= k) { 
            return mid; 
        } 
  
        // If the required element is less than mid 
        else if (countless >= k) { 
            high = mid - 1; 
        } 
  
        // If the required element is greater than mid 
        else if (countless < k 
                 && countless + countequal < k) { 
            low = mid + 1; 
        } 
    } 
} 
  
// Driver code 
int main() 
{ 
    int arr[] = { 7, 10, 4, 3, 20, 15 }; 
    int n = sizeof(arr) / sizeof(int); 
    int k = 3; 
  
    cout << kthSmallest(arr, k, n); 
  
    return 0; 
}