2020-02-19

Author: JiayangWu

1136.平行课程/1136-平行课程.py

@@ -0,0 +1,33 @@
+class Solution(object):
+    def minimumSemesters(self, N, relations):
+        """
+        :type N: int
+        :type relations: List[List[int]]
+        :rtype: int
+        """
+        indegree = [0 for i in range(N + 1)]
+        adj = [set() for _ in range(N + 1)]
+        for pre, cur in relations: 
+            indegree[cur] += 1 #统计入度
+            adj[pre].add(cur) #统计邻居节点
+            
+        from collections import deque
+        queue = deque()
+        for i, x in enumerate(indegree):
+            if x == 0 and i > 0: #将入度为0的节点入队
+                queue.append(i)
+
+        semester_cnt= 0
+        finished_course = 0
+        while queue:
+            next_queue = deque()
+            semester_cnt += 1 #新的学期来了
+            for cur in queue:         
+                finished_course += 1 #又一门课学完了
+                for neighbor in adj[cur]:
+                    indegree[neighbor] -= 1
+ 
+                    if indegree[neighbor] == 0:
+                        next_queue.append(neighbor) #下个学期可以学neighbor这门课了
+            queue = next_queue
+        return semester_cnt if finished_course == N else -1 #如果所有的课都学完了，那么finished_course == N