let 75 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.

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?

主题思想： ； 采用in-place 的方法对三种数字进行排序，思路是：借鉴3路快排，

class Solution {
    public void sortColors(int[] nums) {

        if(nums==null||nums.length==0) return ;

       int p1=0,p2=nums.length-1,index=0;
        while(index<=p2){


            if(nums[index]==0){
                //swap p1 index
                nums[index]=nums[p1];
                nums[p1]=0;
                p1++;
            }
            //swap index p2,  
            if(nums[index]==2){
                nums[index]=nums[p2];
                nums[p2]=2;
                p2--;
                //because p2 has not be handle ,may equal 0,so index need to handle so back index by index--
                //important
                index--;
            }
            index++;
        }
    }
}