[LeetCode 6.11] Sort Colors-优快云博客

本文介绍了一种用于排序红、白、蓝三种颜色对象的算法，通过一通或两通算法实现原地排序，避免使用额外空间。提供了Python实现，包括计数排序的两通算法和仅使用常数空间的一通算法。

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]

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?

Python3 Solution:

two-pass

class Solution:
    def sortColors(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        count0, count1 = 0, 0
        for num in nums:
            if num == 0:
                count0 += 1
            elif num == 1:
                count1 += 1
        for i in range(count0):
            nums[i] = 0
        for i in range(count0, count0+count1):
            nums[i] = 1
        for i in range(count0+count1, len(nums)):
            nums[i] = 2

one-pass

class Solution:
    def sortColors(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        l, r, zero = 0, len(nums)-1, 0
        while l <= r:
            if nums[l] == 0:
                nums[l], nums[zero] = nums[zero], nums[l]
                l += 1
                zero += 1
            elif nums[l] == 2:
                nums[l], nums[r] = nums[r], nums[l]
                r -= 1
            else:
                l += 1