LeetCode 75. Sort Colors



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.


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?


first count appearances of each color. Then replace original array.

Python Solution

class Solution:
    def sortColors(self, nums: List[int]) -> None:
        Do not return anything, modify nums in-place instead.
        colors = [0 for i in range(0, 3)]
        for num in nums:
            colors[num] += 1
        current = 0
        for i in range(0, 3):
            for j in range(0, colors[i]):
                nums[current] = i
                current += 1
  • Time complexity: O(N).
  • Space complexity: O(N).

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