Sort Colors

Problem

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.

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?

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Idea

两个Index,一个指向红色的最后一个(redIndex)， 一个指向蓝色的第一个(blueIndex)， 两个指针分别从两端起始

对于每个A[i] 若A[i]是红色则把他与A[redIndex] 交换并且i 和 redIndex加一

若A[i]是蓝色则把他与A[blueIndex]交换，但是只减blueIndex，i不变。

白色的话则只i++

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Solution

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