Leetcode – Color Sort

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.

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?


Since there are only three kinds of values in the array, we can have three pointers: left and right, and curIndex. 

We keep the following invariants:

We start from left = 0, curIndex = 0, right = A.length – 1

if A[curIndex] < pivot, we exchange it with A[left], now A[left] == 0,  A[curIndex] == 1, to keep the invariance, we let left++ and curIndex++.

if A[curIndex] > pivot, we exchange it with A[right], now A[right] == 2,   then right–

if A[curIndex] == pivot, we just have curIndex++.

Note: This algorithm can also be used as the partition function for quick sort when there are duplicated values for the pivot. 

 The following is the implementation of the color Sort in Java: