Leetcode 75. Sort Colors

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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?

技术图片
 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

 

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