How to implement randomized O(n) algorithm for finding median of unsorted array in Java?

Question

I have this code for finding median of an unsorted array in O(n) expected time, O(n^2) worst case. I use longs because I want to be able to hold long values.

public class Randomized {

    long kthSmallestHelper(long arr[], long l, long r, long k) 
    { 
        if (k > 0 && k <= r - l + 1) 
        { 
            long pos = randomPartition(arr, l, r); 

            if (pos-l == k-1) 
                return arr[(int)pos]; 

            if (pos - l > k - 1) 
                return kthSmallestHelper(arr, l, pos - 1, k);

            return kthSmallestHelper(arr, pos + 1, r, k - pos + l - 1); 
        } 
          return Integer.MAX_VALUE; 
    } 

    void swap(long arr[], long i, long j) 
    { 
        long temp = arr[(int)i]; 
        arr[(int)i] = arr[(int)j]; 
        arr[(int)j] = temp; 
    } 

    long partition(long arr[], long l, long r) 
    { 
        long x = arr[(int)r], i = l; 
        for (long j = l; j <= r - 1; j++) 
        { 
            if (arr[(int)j] <= x) 
            { 
                swap(arr, i, j); 
                i++; 
            } 
        } 
        swap(arr, i, r); 
        return i; 
    } 

    long randomPartition(long arr[], long l, long r) 
    { 
        long n = r - l + 1; 
        long pivot = (long)(Math.random()) * (n - 1); 
        swap(arr, pivot + 1, r); 
        return partition(arr, l, r); 
    } 

    long kthSmallestRandom(long arr[], long k){
        return kthSmallestHelper(arr, 0, arr.length - 1, k);

    }

}

But when I run it

    long[] array =  {12, 3, 5, 7, 4, 19, 26}; //median is 7
    Randomized rand = new Randomized();
    System.out.println(rand.kthSmallestRandom(array, (array.length + 1)/2));

It's incorrect (it returns 4).

My idea was to use this version of the kth smallest number to say that I want the (length/2)th smallest, which is the median.

What is wrong with my idea or implementation?

Answer 1

I don't know why, but just changing all longs to ints fixed it. It now works as expected.