075. Sort Colors

Question 75

https://leetcode.com/problems/sort-colors/description/

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]

Answer

v1.

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.

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        
        Count0 = Count1 = 0
        for i in nums:
            if i == 0:
                Count0 += 1
            elif i == 1:
                Count1 += 1
        
        Count1 += Count0
        for index in range(0,len(nums)):
            if index < Count0:
                nums[index] = 0
            elif index < Count1:
                nums[index] = 1
            else:
                nums[index] = 2

v2.

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

for

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        
        index0, index1, index2 = 0, 0, len(nums)-1
        
        while index1 <= index2:
            if nums[index1] == 0:
                nums[index0], nums[index1] = 0, nums[index0]
                index1 += 1
                index0 += 1
            elif nums[index1] == 2:
                nums[index2], nums[index1] = 2, nums[index2]
                #index1 += 1
                index2 -= 1
            else:
                index1 += 1
            
            #print(index0,index1,index2)
            #print(nums)

while

class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        index0, index1 = 0, 0
        
        for i in range(0, len(nums)):
            v = nums[i]
            nums[i] = 2
            
            if v == 0:
                nums[index1] = 1
                index1 += 1
                nums[index0] = 0
                index0 += 1
            elif v == 1:
                nums[index1] = 1
                index1 += 1