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:
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.
from collections import defaultdict
class Solution(object):
def canFinish(self, numCourses, prerequisites):
"""
:type numCourses: int
:type prerequisites: List[List[int]]
:rtype: bool
"""
record_dict=defaultdict(list)
for item in prerequisites:
key1=item[0]
require=item[1]
record_dict[key1].append(require)
cid=[0]*numCourses
for i in range(numCourses):
if not self.dfs(record_dict, i, cid):
return False
return True
def dfs(self, graph, i, visited):
if visited[i] == 1:
return False
elif visited[i] == 2:
return True
visited[i] = 1
for j in graph[i]:
if not self.dfs(graph, j, visited):
return False
visited[i] = 2
return True