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210. Course Schedule II.py
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210. Course Schedule II.py
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class Solution:
white = 1
gray = 2
black = 3
def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
#Standard textbook topological sort using DFS, color vertices White, Gray, or Black
#LeetCode implementation, time O(V+E), space O(V+E)
graph = collections.defaultdict(list)
for dest, src in prerequisites:
graph[src].append(dest)
ans = []
isPossible = True
color = {k: Solution.white for k in range(numCourses)}
def dfs(node):
nonlocal isPossible
if color[node] == Solution.black:
return
if color[node] == Solution.gray:
isPossible = False
return
color[node] = Solution.gray
for nextNode in graph[node]:
dfs(nextNode)
color[node] = Solution.black
ans.append(node)
for course in range(numCourses):
if color[course] == Solution.white:
dfs(course)
return ans[::-1] if isPossible else []
# #First attempt in failed mock interview, had not yet learned topological sort
# #Second attempt after reading CLRS book section 22.4 on topological sort, time O(V+E), space O(V+E)
# graph = collections.defaultdict(list)
# indegree = [0]*numCourses
# for prereq in prerequisites:
# a, b = prereq
# graph[b].append(a)
# indegree[a] += 1
# queue = collections.deque([i for i in range(numCourses) if indegree[i] == 0])
# ans = []
# while queue:
# course = queue.pop()
# ans.append(course)
# for nextCourse in graph[course]:
# indegree[nextCourse] -= 1
# if indegree[nextCourse] == 0:
# queue.append(nextCourse)
# return ans if len(ans) == numCourses else []