75. Sort Colors -- python

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最新推荐文章于 2025-01-13 09:17:36 发布

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分类专栏： leetcode 文章标签： leetcode python

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本文介绍了一个一通排序算法，用于解决三色问题。通过使用双指针技术，可以在常数空间复杂度内实现原地排序，使相同颜色的对象相邻且按红、白、蓝顺序排列。

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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:

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?

解题：

要求one pass完成排序，可以用双指针left, right来记录当前已经就位的0序列和2序列的边界位置。

class Solution(object):
    def sortColors(self, nums):
        if nums == []:
            return 
        i,l,r = 0,0,len(nums)-1    #定义了两个指针l，r，还有一个index
        while i <= r :
            if nums[i] == 2:       #如果当前index的值等于2
                nums[i],nums[r] = nums[r],nums[i]   #就交换当前index跟右边指针的值
                r -=1                # 然后右指针向左移一位
            elif nums[i] == 0:  #如果当前index的值等于0
                nums[i],nums[l] = nums[l],nums[i]  #就交换当前index跟左边边指针的值
                l +=1
                i +=1         # 然后左指针向右移一位，index也右移
            else :
                i+=1       #如果当前index的值等于1，那index右移，左指针不变