Leetcode 75. Sort Colors
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https://leetcode.com/problems/sort-colors/
Medium
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]
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?
- Dutch national flag problem。类似快排的three-way-partition。用三个指针,保证0 <= i <= j <= n <= len(array) - 1,i 是小于middle的元素的最大下标,n是大于middle的元素的最小下标,j是当前元素下标。最终划分成 [0, i], (i, n), [n, len(array) - 1]。
- Python O(n) 1 pass in-place solution with explanation - LeetCode Discuss
- Dutch national flag problem - Wikipedia
1 class Solution: 2 def sortColors(self, nums: List[int]) -> None: 3 """ 4 Do not return anything, modify nums in-place instead. 5 """ 6 7 def three_way_partition(nums, middle): 8 i, j, n = 0, 0, len(nums) - 1 9 10 while j <= n: 11 if nums[j] < middle: 12 nums[i], nums[j] = nums[j], nums[i] 13 i += 1 14 j += 1 15 elif nums[j] > middle: 16 nums[j], nums[n] = nums[n], nums[j] 17 n -= 1 18 else: 19 j += 1 20 21 three_way_partition(nums, 1)
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