SortColors.java

public void sortColors(int[] A) {
    if (A == null || A.length == 0) return;

    int left = 0;
    int right = A.length - 1;

    int i = 0; 
    while (i <= right) { // has to be <=, since cur value still needs to be evaluated if swapped back
        if (A[i] == 1) {
            i++;
        } else if (A[i] == 0) {
            swap(A, i, left);
            left++;
            i++; // don't forget this one, we can only exchange 0 or 1 back here, so move forward no matter what
        } else { // A[i] == 2
            swap(A, i, right);
            right--;
        }
    }
}