Follow up:
A rather straight forward solution is a two-pass algorithm using counting sort.
First, iterate the array counting number of 0‘s, 1‘s, and 2‘s, then overwrite array with total number of 0‘s, then 1‘s and followed by 2‘s.

Could you come up with an one-pass algorithm using only constant space?

public class Solution {
    public void sortColors(int[] A) {
        int i=-1;
        int j=A.length;
        int middle=0;
        while(middle<j)
        {
            if(A[middle]==0)
            {
                i++;
                middle++;
                A[i]=0;
            }
            else{
                if(A[middle]==1)
                {
                    middle++;
                }
                else
                {
                    j--;
                    while(middle<j&&A[j]==2)
                    {
                        j--;
                    }
                    int t=A[j];
                    A[j]=A[middle];
                    A[middle]=t;
                }
            }
        }
        for(middle=i+1;middle<j;middle++)
        {
            A[middle]=1;
        }
    }
}