Sort Colors - Python Leetcode

 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?

class Solution:

    def sortColors(self,nums: List[int]) -> None:

        """ 

        Dutch National Flag Problem

        """

        # For all idx nums[idx < p0] == 0 and idx < p0

        # For all idx idx > p2 : nums[idx > p2] == 2

        # curr is pointing current element

        

        if not nums:

            return 

        curr = p0 = 0

        p2 = len(nums) -1

        

        while curr <= p2:

            if nums[curr] == 0:

                nums[p0],nums[curr] = nums[curr],nums[p0]

                curr = curr+1

                p0 += 1 

            elif nums[curr] == 2:

                nums[curr],nums[p2] = nums[p2],nums[curr]

                p2 -= 1

            else:

                curr+=1