Leetcode 207. Course Schedule (topological sort)

Topic

There is a total of n courses you have to take, labeled from 0 to n - 1 .

Some courses May has 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, are it possible for your to finish all courses?

For example:

2, [[1,0]]

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

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

There is a total of 2 courses to take. To take course 1 should has finished course 0, and to take course 0 you should also has finished course 1. So it is impossible.

Note:

1. The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more on how a graph is represented.
2. Assume that there is no duplicate edges in the input prerequisites.

Thinking of solving problems

1. The essence is a topological sort problem.
2. Calculate and store the degrees and degrees of each task
3. Place a zero-in task in a queue
4. The 0-in queue is continuously ejected, and the out-of-order relationship is maintained (removed), and when the task's zero-in degree is maintained, the queue is queued until it is empty.

Class solution (Object): Def canfinish (self, numcourses, prerequisites): "" ": Type Numcourses:int : Type Prerequisites:list[list[int]]: Rtype:bool "" "# Create in-degree out-of-degrees empty dictionary indegree, Outdegree = {x:[] For x in range (numcourses)}, {x:[] for x in range (numcourses)} # stores the in-degree-out relationship for course, precourse in P                    Rerequisites:indegree[course].append (Precourse) outdegree[precourse].append (course) count, Emptyqueue = 0, [] # The task of entering zero into the queue for I in range (numcourses): If not indegree. Get (i): Emptyqueue.append (i) # Popup task, maintenance task 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 = = Numcou                 RSes

　　

　　

Leetcode 207. Course Schedule (topological sort)