75. Sort Colors

https://leetcode.com/problems/sort-colors/#/description

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.

Sol:

Like Question "Move Zeros".  Overwrite out of place elements using counter. 

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        # Just like the Lomuto partition algorithm usually used in quick sort. We keep a loop invariant that [0,i) [i, j) [j, k) are 0s, 1s and 2s sorted in place for [0,k). Here ")" means exclusive. We don't need to swap because we know the values we want.
        # swap is confusing, just write a new sorted list in place.
        
        i = j = 0
        for k in range(len(nums)):
            v = nums[k]
            nums[k] = 2
            if v < 2:
                nums[j] = 1
                j += 1
            if v == 0 :
                nums[i] = 0
                i += 1