# 207. Course Schedule

## Medium

***

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**.

```python
from collections import defaultdict
class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        degree = [0]*numCourses
        d = defaultdict(list)
        q = deque()
        ans = []
        for prev,nxt in prerequisites:
            degree[nxt] += 1
            d[prev].append(nxt)
        for index in range(numCourses):
            if degree[index] == 0:
                q.append(index)
        while q:
            course = q.pop()
            ans.append(course)
            for adjCourse in d[course]:
                degree[adjCourse] -= 1
                if degree[adjCourse] == 0:
                    q.append(adjCourse)
        return True if len(ans) == numCourses else False
```