# LeetCode 75. Sort Colors

## Description

https://leetcode.com/problems/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]```

• 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?

## 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).