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?


1. Two Pointers

  左指针记录当前已知的 0 排好序后最右边的位置,右指针记录当前已知的 2,排好序后最左边的位置。然后更新左右指针,直到计数器从 0 遍历到右指针的位置。

# Time: O(n)
# Space: O(1)
class Solution:
    def sortColors(self, nums: List[int]) -> None:
        Do not return anything, modify nums in-place instead.
        n = len(nums)
        l, r = -1, n
        i = 0
        while i < r:
            if nums[i] == 0:
                l += 1
                nums[i], nums[l] = nums[l], nums[i]
                i += 1
            elif nums[i] == 2:
                r -= 1
                nums[i], nums[r] = nums[r], nums[i]
                i += 1
        return nums
# Runtime: 28 ms, faster than 98.10% of Python3 online submissions for Sort Colors.
# Memory Usage: 12.7 MB, less than 100.00% of Python3 online submissions for Sort Colors.


  1. 75. Sort Colors
  2. Four different solutions