Tags: "leetcode", "python", access_time 1-min read

Sort Colors

Created: November 7, 2018 by [lek-tin]

Last updated: November 7, 2018

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

1. 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.
2. Could you come up with a one-pass algorithm using only constant space?

Solution

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        if len(nums) == 0 and nums == None:
            return

        def swap(nums, i, j):
            temp = nums[i]
            nums[i] = nums[j]
            nums[j] = temp

        left, index, right = 0, 0, len(nums) - 1

        while index <= right:
            if nums[index] == 0:
                swap(nums, index, left)
                index += 1
                left += 1
            elif nums[index] == 1:
                index += 1
            else:
                swap(nums, index, right)
                right -= 1