Leet_Code/027_course_schedule.py at master · Kcheung42/Leet_Code · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
#------------------------------------------------------------------------------
# Question: 027_course_schedule.py
#------------------------------------------------------------------------------
# tags: medium
'''
There are a total of n courses you have to take, labeled from 0 to n-1.

Some courses may have prerequisites, for example to take course 0 you have to
first take course 1, which is expressed as a pair: [0,1]

Given the total number of courses and a list of prerequisite pairs, is it
possible for you to finish all courses?

Example 1:

Input: 2, [[1,0]]
Output: true
Explanation: There are a total of 2 courses to take. To take course 1 you should
have finished course 0. So it is possible.

Example 2:

Input: 2, [[1,0],[0,1]]
Output: false
Explanation: There are a total of 2 courses to take. To take course 1 you should
have finished course 0, and to take course 0 you should also have finished course
1. So it is impossible.

'''

#------------------------------------------------------------------------------
# Solutions
#------------------------------------------------------------------------------
from typing import *
from test_utils.debug import debug
import collections


class Solution:
    '''
    Time:
    '''


#     def canFinish(self, numCourses, prerequisites):

#         # @debug
#         def dfs(course):
#             visited[course] = True
#             for nei in graph[course]:
#                 if visited[nei] == False:
#                     dfs(nei)
#             stack.append(course)

#         visited = [False] * numCourses
#         graph = collections.defaultdict(set)
#         stack = []
#         for course, pre in prerequisites:
#             graph[course].add(pre)
#         print(graph)

#         # check for cycles

#         for i in range(numCourses):
#             if visited[i] == False:
#                 dfs(i)
#         print(stack)
#         return(len(stack) == numCourses)


class SolutionLeet:
    '''
    Time:
    Space:
    Intuition: Topological Sort
    '''
    def canFinish(self, numCourses, prerequisites):
        courses = collections.defaultdict(set)
        prereqs = collections.defaultdict(set)
        for course, pre in prerequisites:
            courses[course].add(pre)
            prereqs[pre].add(course)

        #get all nodes that don't have a prerequisite
        stack = [n for n in range(numCourses) if not courses[n]]
        '''
        0 > 1 > 2
            ^___|
        '''
        print(f'\n courses: {courses}')
        print(f'\n neighbors: {prereqs}')
        count = 0
        print(f'\n starting stack: {stack}')
        while stack:
            print(f'\n stack: {stack}')
            course = stack.pop()
            # print(f'\nnode: {course}')

            count += 1
            # print(f'\ncount: {count}')

            for p in prereqs[course]:
                courses[p].remove(course)
                # course does not have any pre-reqs to process
                if not courses[p]:
                    stack.append(p)
        # print(f'\ncount: {count}')

        return count == numCourses


#------------------------------------------------------------------------------
# Tests
#------------------------------------------------------------------------------
import unittest


class TestSolution(unittest.TestCase):
    def test_simple_true(self):
        numsCourses = 2
        prereqs = [[0, 1]]
        s = SolutionLeet()
        self.assertEqual(s.canFinish(numsCourses, prereqs), True)

        prereqs = [[1, 0], [0, 1]]
        s = SolutionLeet()
        self.assertEqual(s.canFinish(numsCourses, prereqs), False)

    def test_simple_false2(self):
        '''
        1 > 0
        ^___|
        '''
        numsCourses = 2
        prereqs = [[0, 1], [1, 0]]
        s = SolutionLeet()
        self.assertEqual(s.canFinish(numsCourses, prereqs), False)

    def test_simple_false3(self):
        '''
        0 > 1 > 2
        ^_______|
        '''
        numsCourses = 3
        prereqs = [[2, 1], [1, 0], [0, 2]]
        s = SolutionLeet()
        self.assertEqual(s.canFinish(numsCourses, prereqs), False)


    def test_simple_false4(self):
        '''
        0 > 1 > 2
            ^___|
        '''
        numsCourses = 3
        prereqs = [[2, 1], [1, 0], [1, 2]]
        s = SolutionLeet()
        self.assertEqual(s.canFinish(numsCourses, prereqs), False)

    def test_two_classes_require_one(self):
        numsCourses = 3
        prereqs = [[0, 1], [0, 2], [1, 2]]
        s = SolutionLeet()
        self.assertEqual(s.canFinish(numsCourses, prereqs), True)

    def test_circular(self):
        '''
        graph:
        0 : []
        1 : [0,4]
        2 : [1]
        3 : [2]
        4 : [0]
        5 : []

        Cycle exists

                3
                ^
        0 > 1 > 2
        ^   |   |
        4<--*   |
        ^-------*
        '''

        numsCourses = 6
        prereqs = [[1, 0], [2, 1], [3, 2], [4, 1], [0, 4], [2, 4]]
        s = SolutionLeet()
        self.assertEqual(s.canFinish(numsCourses, prereqs), False)


unittest.main(verbosity=2)