LeetCode 207. Course Schedule（拓扑排序）

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?

For example:

2, [[1,0]]

There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible.

2, [[1,0],[0,1]]

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.

Note:

The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.

You may assume that there are no duplicate edges in the input prerequisites.

class Solution(object):
def canFinish(self, numCourses, prerequisites):
"""
:type numCourses: int
:type prerequisites: List[List[int]]
:rtype: bool
"""
# 创建入度出度空字典
inDegree, outDegree = {x:[] for x in range(numCourses)}, {x:[] for x in range(numCourses)}

# 存储入度出度关系
for course, preCourse in prerequisites:
inDegree[course].append(preCourse)
outDegree[preCourse].append(course)

count, emptyQueue = 0, []

# 将入度为零的任务加入队列
for i in range(numCourses):
if not inDegree.get(i):
emptyQueue.append(i)

# 弹出任务，维护任务
while emptyQueue:
node = emptyQueue.pop()
count += 1
for j in outDegree[node]:
inDegree[j].remove(node)
if not inDegree.get(j):
emptyQueue.append(j)

return count == numCourses