Python solution for 207

Author: gavinfish

python/207 Course Schedule.py

@@ -0,0 +1,50 @@
+'''
+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?
+
+Example 1:
+
+Input: 2, [[1,0]] 
+Output: true
+Explanation: There are a total of 2 courses to take. 
+             To take course 1 you should have finished course 0. So it is possible.
+Example 2:
+
+Input: 2, [[1,0],[0,1]]
+Output: false
+Explanation: 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:
+
+    1. The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
+    2. You may assume that there are no duplicate edges in the input prerequisites.
+'''
+
+
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: [[]]) -> bool:
+        graph = {n: [] for n in range(numCourses)}
+        for x, y in prerequisites:
+            graph[x].append(y)
+
+        for target in range(numCourses):
+            stack = graph[target]
+            visited = set()
+            while stack:
+                course = stack.pop()
+                visited.add(course)
+                if course == target:
+                    return False
+                for i in graph[course]:
+                    if i not in visited:
+                        stack.append(i)
+        return True
+
+
+if __name__ == "__main__":
+    assert True == Solution().canFinish(2, [[1, 0]])
+    assert False == Solution().canFinish(2, [[1, 0], [0, 1]])