问题描述：

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.

基本思想：

遍历一遍记录0,1,2的个数，然后利用计数排序写出代码。

代码：

void sortColors(int A[], int n) {  //C++
        int rNum =0,wNum = 0, bNum =0; 
        for(int i = 0; i < n; i++){ 
            if(A[i] == 0) 
                rNum++; 
            else if(A[i] == 1) 
                wNum++; 
            else bNum++; 
        } 
         
        int rBegin = 0;  
        int wBegin = rNum;  
        int bBegin = rNum+wNum; 
         
        //ith element i < rNum  
        for(int i = 0; i < n; i++){ 
            if(i < rNum) 
                A[i] =0; 
            else if(i < n-bNum) 
                A[i] = 1; 
            else A[i] =2; 
        } 
}

one-pass algorithm

void sortColors(int[] A) {


    int i=-1, j=-1, k=-1;

    for(int p = 0; p < A.length; p++)
    {
        if(A[p] == 0)
        {
            A[++k]=2;
            A[++j]=1;
            A[++i]=0;
        }
        else if (A[p] == 1)
        {
            A[++k]=2;
            A[++j]=1;

        }
        else if (A[p] == 2)
        {
            A[++k]=2;
        }
    }

}

[leetcode]Sort Colors