75. Sort Colors

Description

Given an array nums 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.

We will use the integers 0, 1, and 2 to represent the color red, white, and blue, respectively.

You must solve this problem without using the library's sort function.

 

Example 1:

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

Example 2:

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

 

Constraints:

• n == nums.length
• 1 <= n <= 300
• nums[i] is either 0, 1, or 2.

 

Follow up: Could you come up with a one-pass algorithm using only constant extra space?

Solution

sort-colors.py

class Solution:
    def sortColors(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """

        left, right = 0, len(nums) - 1
        i = 0

        while i <= right:
            if nums[i] == 0:
                if nums[left] != 0:
                    nums[left], nums[i] = nums[i], nums[left]

                left += 1
                i += 1
            elif nums[i] == 2:
                if nums[right] != 2:
                    nums[right], nums[i] = nums[i], nums[right]

                right -= 1
            else:
                i += 1