Leetcode 75. Sort Colors

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

Medium

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?

---

• Dutch national flag problem。相似快排的three-way-partition。用三個指針，保證0 <= i <= j <= n <= len(array) - 1，i 是小於middle的元素的最大下標，n是大於middle的元素的最小下標，j是當前元素下標。最終劃分紅 [0, i], (i, n), [n, len(array) - 1]。
• Python O(n) 1 pass in-place solution with explanation - LeetCode Discuss
  • https://leetcode.com/problems/sort-colors/discuss/26481/Python-O(n)-1-pass-in-place-solution-with-explanation
• Dutch national flag problem - Wikipedia
  • https://en.wikipedia.org/wiki/Dutch_national_flag_problem

 1 class Solution:
 2     def sortColors(self, nums: List[int]) -> None:
 3         """
 4         Do not return anything, modify nums in-place instead.
 5         """
 6         
 7         def three_way_partition(nums, middle):
 8             i, j, n = 0, 0, len(nums) - 1
 9             
10             while j <= n:
11                 if nums[j] < middle:
12                     nums[i], nums[j] = nums[j], nums[i]
13                     i += 1
14                     j += 1
15                 elif nums[j] > middle:
16                     nums[j], nums[n] = nums[n], nums[j]
17                     n -= 1
18                 else:
19                     j += 1
20         
21         three_way_partition(nums, 1)

View Python Code