Sort Colors in Python

Suppose we have an array with n objects. These are colored red, white, or blue, sort them in place so that the objects of the same color are adjacent. So with the colors in the order red, white and blue. Here, we will use the numbers like 0, 1, and 2 to represent the color red, white, and blue respectively. So if the array is like [2,0,2,1,1,0], then the output will be [0,0,1,1,2,2]

To solve this, we will follow these steps −

• set low := 0, mid := 0 and high := length of array – 1
• while mid <= high
  • if arr[mid] = 0, then swap arr[mid] and arr[low], and increase low and mid by 1
  • otherwise when arr[mid] = 2, swap arr[high] and arr[mid], decrease high by 1
  • else increase mid by 1

Example(Python)

Let us see the following implementation to get a better understanding −

 Live Demo

class Solution(object):
   def sortColors(self, nums):
      low = 0
      mid = 0
      high = len(nums)-1
      while mid<=high:
         if nums[mid] == 0:
            nums[low],nums[mid] = nums[mid],nums[low]
            low+=1
            mid += 1
         elif nums[mid] == 2:
            nums[high], nums[mid] = nums[mid], nums[high]
            high-=1
         else:
            mid += 1
      return nums
ob1 = Solution()
print(ob1.sortColors([2,0,2,1,1,0]))

Input

[2,0,2,1,1,0]

Output

[0,0,1,1,2,2]

Arnab Chakraborty

Updated on: 27-Apr-2020

2K+ Views

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