# LeetCode 207. Course Schedule

## Description

https://leetcode.com/problems/course-schedule/

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 course 0 you have to first take course 1.

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 <= 105
• 0 <= prerequisites.length <= 5000
• prerequisites[i].length == 2
• 0 <= ai, bi < numCourses
• All the pairs prerequisites[i] are unique.

## Explanation

Use topological sort to keep appending the course with indegree 0 to the queue. In the end, check how many number of courses can be taken.

## Python Solution

class Solution:
def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:

indegrees = []

for i in range(numCourses):
indegrees.append(0)

for prerequisite in prerequisites:
indegrees[prerequisite[1]] += 1

queue = []

for i, indegree in enumerate(indegrees):
if indegree == 0:
queue.append(i)

count = 0
while queue:
course = queue.pop(0)
count += 1