Quick Sort

public class Solution {
    /**
     * @param A an integer array
     * @return void
     */
    public void sortIntegers(int[] A) {
        // Write your code here
        if (A == null || A.length == 0) {
            return;
        }
        quickSort(A, 0, A.length - 1);
    }
    
    private void quickSort(int[] A, int start, int end) {
        int pivot = A[start + (end - start) / 2];
        int index = partition(A, pivot, start, end);
        if (start < index - 1) {
            quickSort(A, start, index - 1);
        }
        if (index < end) {
            quickSort(A, index, end);
        }
    }
    
    private int partition(int[] A, int pivot, int start, int end) {
        while (start <= end) {
            while (start <= end && A[start] < pivot) {
                start++;
            }
            while (start <= end && A[end] > pivot) {
                end--;
            }
            if (start <= end) {
                swap(A, start, end);
                start++;
                end--;
            }
        }
        return start;
    }
    
    private void swap(int A[], int i, int j) {
        int temp = A[i];
        A[i] = A[j];
        A[j] = temp;
    }
}