LeetCode - Sort Colors

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.

思路分析：这题很简单，但是要做到O(N)的时间复杂度，并且只用constant 内存不容易，O（N）解决方案有桶排序和Counting Sort，前者需要O（N）的额外空间，后者只需要constant内存。这里给出基于Counting Sort的AC code。

public class Solution {
    public void sortColors(int[] A) {
        //Implement counting sort
        int count1 = 0; 
        int count2 = 0; 
        int count0 = 0;
        for(int i = 0; i < A.length; i++){
            if(A[i] == 2){
                count2++;
            } else if(A[i] == 1){
                count1++;
                swap(A, i , i-count2);
            } else {
                count0++;
                swap(A, i, i-count2);
                swap(A, i-count2, i-count2-count1);
            }
        }
    }
    
    public void swap(int[] A, int i, int j){
        int temp;
        temp = A[i];
        A[i] = A[j];
        A[j] = temp;
    }
}