207. Course Schedule

README.md

@@ -22,6 +22,7 @@ LeetCode の問題を以下手順で解く
 
 | #   | Title                                                                                                 | Language | Difficulty |
 | --- | ----------------------------------------------------------------------------------------------------- | -------- | ---------- |
+| 207 | [Course Schedule](https://leetcode.com/problems/course-schedule/description/)                         | Python3  | Medium     |
 | 322 | [Coin Change](https://leetcode.com/problems/coin-change/description/)                                 | Python3  | Medium     |
 | 417 | [Pacific Atlantic Water Flow](https://leetcode.com/problems/pacific-atlantic-water-flow/description/) | Python3  | Medium     |
 

problems/medium/python3/207/course-schedule.step1.py

@@ -0,0 +1,35 @@
+#
+# @lc app=leetcode id=207 lang=python3
+#
+# [207] Course Schedule
+#
+
+# @lc code=start
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
+        constraint_map, num_of_constraint_map = dict(), dict()
+        for after, before in prerequisites:
+            if before not in constraint_map:
+                constraint_map[before] = []
+            constraint_map[before].append(after)
+            if after not in num_of_constraint_map:
+                num_of_constraint_map[after] = 0
+            num_of_constraint_map[after] += 1
+
+        no_constraint_courses = []
+        for i in range(numCourses):
+            if i not in num_of_constraint_map:
+                no_constraint_courses.append(i)
+
+        taking_order = []
+        while no_constraint_courses:
+            course = no_constraint_courses.pop()
+            taking_order.append(course)
+            if course not in constraint_map:
+                continue
+            for constraint in constraint_map[course]:
+                num_of_constraint_map[constraint] -= 1
+                if num_of_constraint_map[constraint] == 0:
+                    no_constraint_courses.append(constraint)
+        return len(taking_order) == numCourses
+# @lc code=end

problems/medium/python3/207/course-schedule.step2.py

@@ -0,0 +1,31 @@
+#
+# @lc app=leetcode id=207 lang=python3
+#
+# [207] Course Schedule
+#
+
+# @lc code=start
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
+        postrequisites_of_courses = [[] for _ in range(numCourses)]
+        num_prerequisites_of_courses = [0 for _ in range(numCourses)]
+
+        for course, prerequisite in prerequisites:
+            postrequisites_of_courses[prerequisite].append(course)
+            num_prerequisites_of_courses[course] += 1
+
+        independent_courses = []
+        for i in range(numCourses):
+            if num_prerequisites_of_courses[i] == 0:
+                independent_courses.append(i)
+
+        num_of_processed_courses = 0
+        while independent_courses:
+            ic = independent_courses.pop()
+            for postrequisites in postrequisites_of_courses[ic]:
+                num_prerequisites_of_courses[postrequisites] -= 1
+                if num_prerequisites_of_courses[postrequisites] == 0:
+                    independent_courses.append(postrequisites)
+            num_of_processed_courses += 1
+        return num_of_processed_courses == numCourses
+# @lc code=end

problems/medium/python3/207/course-schedule.step3.py

@@ -0,0 +1,32 @@
+#
+# @lc app=leetcode id=207 lang=python3
+#
+# [207] Course Schedule
+#
+
+# @lc code=start
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
+        postrequisites_of_courses = [[] for _ in range(numCourses)]
+        for course, prerequisite in prerequisites:
+            postrequisites_of_courses[prerequisite].append(course)
+
+        NOT_CHECKED, CHECKING, COMPLETED = 0, 1, 2
+        def hasCycle(labels: List[int], course: int) -> bool:
+            if labels[course] == CHECKING:
+                return True
+            if labels[course] == COMPLETED:
+                return False
+            labels[course] = CHECKING
+            for post in postrequisites_of_courses[course]:
+                if hasCycle(labels, post):
+                    return True
+            labels[course] = COMPLETED
+            return False
+
+        labels = [NOT_CHECKED for _ in range(numCourses)]
+        for course in range(numCourses):
+            if hasCycle(labels, course):
+                return False
+        return True
+# @lc code=end