Description
There are a total of numCourses
courses you have to take, labeled from 0
to numCourses - 1
. You are given an array prerequisites
where prerequisites[i] = [ai, bi]
indicates that you must take course bi
first if you want to take course ai
.
- For example, the pair
[0, 1]
, indicates that to take course0
you have to first take course1
.
Return true
if you can finish all courses. Otherwise, return false
.
Example 1:
Input: numCourses = 2, prerequisites = [[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: numCourses = 2, prerequisites = [[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.
Constraints:
1 <= numCourses <= 2000
0 <= prerequisites.length <= 5000
prerequisites[i].length == 2
0 <= ai, bi < numCourses
- All the pairs prerequisites[i] are unique.
Solution
Python3
class Solution:
def canFinish(self, n: int, prerequisites: List[List[int]]) -> bool:
indegree = [0] * n
graph = defaultdict(list)
took = 0
for a, b in prerequisites:
graph[b].append(a)
indegree[a] += 1
queue = deque([i for i in range(n) if indegree[i] == 0])
while queue:
node = queue.popleft()
took += 1
for adj in graph[node]:
indegree[adj] -= 1
if indegree[adj] == 0:
queue.append(adj)
return took == n