LeetCode 75. Sort Colors

Description

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

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]

Example 3:

Input: nums = [0]
Output: [0]

Example 4:

Input: nums = [1]
Output: [1]

Constraints:

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

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

Explanation

first count appearances of each color. Then replace original array.

Python Solution

class Solution:
    def sortColors(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        colors = [0 for i in range(0, 3)]
        
        for num in nums:
            colors[num] += 1
            
        current = 0
        for i in range(0, 3):
            for j in range(0, colors[i]):
                nums[current] = i
                current += 1
        

• Time complexity: O(N).
• Space complexity: O(N).