Sort color

Given an array with n objects colored red, white or blue, sort them so that objects of the same color are adjacent, with the colors in the order red, white and blue.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note:
You are not suppose to use the library's sort function for this problem.

I have solved this four times, but I still couldn't remember it correctly. It is similar to dual pivot quick sort.
1. We take two pivots, the left one (pl) initialized at index 0 and the right one initialized at index A.length - 1;
2. Then we loop the array, if A[index] == 0, we swap it with pl, increment pl; if A[index] == 2, we swap it with pr, decrement pr.

public void sortColors(int[] A) {
        if (A == null || A.length < 2)
            return;
        int pl = 0;
        int pr = A.length - 1;
        int index = 0;
        while (index <= pr){
            if (A[index] == 0){
                swap(A, index++, pl++);
            }
            else if (A[index] == 2)
                swap(A, index, pr--);
            else
                index++;
        }
    }
    private void swap(int[] A, int i, int j){
        int tmp = A[i];
        A[i] = A[j];
        A[j] = tmp;
    }