LeetCode 75. Sort Colors

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题目

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:

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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?

思路

三路快排。

代码

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