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]
- 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.
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).