[LeetCode] 75. Sort Colors_Medium tag: Two pointers

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?

 

这个题目就是像利用quick sort，设一个mid，然后利用two pointers分别将小于等于mid的放到左边，大于mid的放到右边，先将0放在左边，然后再将mid设为1用一遍，即可。

Code

class Solution:
    def sortColors(self, nums):
    """
        Do not return anything, modify nums in-place instead.
    """
        def divideMid(nums, start, end, mid):
            while start < end:
                while start < end and nums[start] <= mid:
                    start += 1
                while start < end and nums[end] > mid:
                    end -= 1
                if start < end:
                    nums[start], nums[end] = nums[end], nums[start]
                    start += 1
                    end -= 1
            return start
        n = len(nums)
        index = divideMid(nums, 0, n - 1, 0)
        divideMid(nums, index, n - 1, 1)