Find Kth smallest element in an array

Quick select

public static int selectKth(int[] arr, int k) {
    if (arr == null || arr.length <= k)
        throw new Error();

    int from = 0, to = arr.length - 1;

    // if from == to we reached the kth element
    while (from < to) {
        int r = from, w = to;
        int mid = arr[(r + w) / 2];

        // stop if the reader and writer meets
        while (r < w) {

            if (arr[r] >= mid) { // put the large values at the end
                int tmp = arr[w];
                arr[w] = arr[r];
                arr[r] = tmp;
                w--;
            } else { // the value is smaller than the pivot, skip
                r++;
            }
        }

        // if we stepped up (r++) we need to step one down
        if (arr[r] > mid)
            r--;

        // the r pointer is on the end of the first k elements
        if (k <= r) {
            to = r;
        } else {
            from = r + 1;
        }
    }
    return arr[k];
}