Sort Colors [Leetcode]

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.

click to show follow up.

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?

Solution: the key point is to sort it using an one-pass algorithm. The basic idea is inspired by the PARTITION algorithm in QuickSort.

class Solution {
public:
    void swap(int &a, int &b){
        int tmp = a;
        a = b;
        b = tmp;
    }
    void sortColors(int A[], int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        if(n<0) return;
        int i=-1, j=-1;
        for(int k=0; k<n; ++k){
            if(A[k]==0){
                swap(A[++j], A[k]);
                swap(A[++i], A[j]);
            }
            else if(A[k]==1){
                swap(A[++j], A[k]);
            }
        }
    }
};