Leetcode解题-Sort Colors

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描述

Given an array with n objects colored red, white or blue, sort them 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.
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 an one-pass algorithm using only constant space?

分析

使用计数排序很简单。题目进阶要求只扫一遍数组，就有点意思了。可以用两指针左右夹逼，遇到0就放到前面，遇到2就放到后面，比较像快排中的partition。

代码

计数排序

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class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        counts = [0, 0, 0]
        for x in nums:
            counts[x] += 1
        i = 0
        for j in xrange(len(counts)):
            for k in xrange(counts[j]):
                nums[i] = j
                i += 1

两指针

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class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        pivot = 1
        i = 0
        j = len(nums) - 1
        k = i
        while k <= j:
            if nums[k] < pivot:
                if k == i:
                    k += 1
                    i += 1
                else:
                    nums[k], nums[i] = nums[i], nums[k]
                    i += 1
            elif nums[k] > pivot:
                nums[k], nums[j] = nums[j], nums[k]
                j -= 1
            else:
                k += 1

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