1 """
2 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.
3 Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.
4 Note: You are not suppose to use the library's sort function for this problem.
5 Example:
6 Input: [2,0,2,1,1,0]
7 Output: [0,0,1,1,2,2]
8 """
9 """
10 绝世好题：提供了四种解法
11 传送门：https://blog.csdn.net/qq_17550379/article/details/80499816
12 1.计数排序
13 2.三路快排(回顾liuyubobobo的视频第三章6-9)
14 3.适合此题的三路排序算法
15 4.三指针
16 """
17 """
18 解法一：计数排序
19 """
20 class Solution1:
21 def sortColors(self, nums):
22 """
23 Do not return anything, modify nums in-place instead.
24 """
25 count = [0, 0, 0]
26 for i in nums:
27 assert(i >= 0 and i <= 2)
28 #python assert断言是声明布尔值必须为真的判定，如果发生异常就说明表达式为假。
29 #可以理解assert断言语句为raise-if-not，用来测试表示式，其返回值为假，就会触发异常。
30 count[i] += 1
31 index = 0
32 for i, num in enumerate(count):
33 for j in range(num):
34 nums[index] = i
35 index += 1
36
37 """
38 解法二：快速排序(三路)，没有随机标兵点
39 """
40 class Solution2:
41 def sortColors(self, nums: List[int]) -> None:
42 """
43 Do not return anything, modify nums in-place instead.
44 """
45 self._sortColors(nums, 0, len(nums)-1) #bug 没写-1
46 def _sortColors(self, nums, l, r):
47 if r <= l:
48 return
49 pivot = l #标定点
50 lt = l #[l+1, lt] <val
51 i = l+1 #[lt+1,gt-1] = val
52 gt = r+1 #[gt, r] > val
53 while i < gt:
54 if nums[i] < nums[pivot]:
55 lt += 1
56 nums[i], nums[lt] = nums[lt], nums[i]
57 i += 1
58 elif nums[i] > nums[pivot]:
59 gt -= 1
60 nums[i], nums[gt] = nums[gt], nums[i] #注意与后面的交换不需要再加i
61 else:
62 i += 1
63 nums[pivot], nums[lt] = nums[lt], nums[pivot]
64 self._sortColors(nums, l, lt-1) #快排交换放在了lt的位置
65 self._sortColors(nums, gt, r)
66
67 """
68 解法三：适合此题的三路排序算法
69 """
70 class Solution3:
71 def sortColors(self, nums):
72 """
73 Do not return anything, modify nums in-place instead.
74 """
75 zero = -1
76 i = 0 #从0开始向后遍历
77 two = len(nums)
78 while i < two:
79 if nums[i] == 1:
80 i += 1
81 elif nums[i] == 2:
82 two -= 1
83 nums[two], nums[i] = nums[i], nums[two]
84 else:
85 zero += 1
86 nums[zero], nums[i] = nums[i], nums[zero]
87 i += 1 #!!! forget
88 """
89 解法四：用三个指针
90 i，j，k分别指向最后一个0，1，2
91 """
92 class Solution4:
93 def sortColors(self, nums):
94 """
95 Do not return anything, modify nums in-place instead.
96 """
97 i, j, k = -1, -1, -1 #i，j，k分别指向最后一个0，1，2
98 for m in range(len(nums)):
99 if nums[m] == 0:
100 k += 1 #bug python中写成 nums[++k] 是无效的
101 nums[k] = 2
102 j += 1
103 nums[j] = 1
104 i += 1
105 nums[i] = 0
106 elif nums[m] == 1:
107 k += 1
108 nums[k] = 2
109 j += 1
110 nums[j] = 1
111 else:
112 k += 1
113 nums[k] = 2
