75-Sort Colors

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题意

Given an array nums 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. Follow up:

• Could you solve this problem without using the library’s sort function?
• Could you come up with a one-pass algorithm using only O(1) constant space?

Example 1:
Input: nums = [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

Example 2:
Input: nums = [2,0,1]
Output: [0,1,2]

Example 3:
Input: nums = [0]
Output: [0]

Example 4:
Input: nums = [1]
Output: [1]

思路

• 特殊排序
• 三指针: start记录0的位置，end记录2的位置 -> 遇0swap到start, 遇2swap到end

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: None Do not return anything, modify nums in-place instead.
        """
        if not nums: return []
        start, end = 0, len(nums)-1
        i = 0
        while i <= end:
            if nums[i] == 0:
                nums[i],nums[start] = nums[start],nums[i]
                i += 1
                start += 1
            elif nums[i] ==1:
                i += 1
            else:
                nums[i],nums[end] = nums[end],nums[i] #此处不可i++, 否则将未排序元素置换到前面
                end -= 1

分析:

• Time: O(n)
• Space: O(1)