75. Sort Colors

Given an array with n objects colored red, white or blue, sort them in-place **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.

Example:

Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

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 a one-pass algorithm using only constant space?

Code：

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: None Do not return anything, modify nums in-place instead.
        """
        red, white, blue = 0, 0, len(nums)-1
        while white <= blue:
            if nums[white] == 0:
                nums[red], nums[white] = nums[white], nums[red]
                white += 1
                red += 1
            elif nums[white] == 1:
                white += 1
            else:
                nums[white], nums[blue] = nums[blue], nums[white]
                blue -= 1