leetcode/apps/docs/content/(04-relational)/topological-sort/207-course-schedule.mdx

---
title: '207. Course Schedule'
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
icon: dot
topics:
  - { name: "Depth-First Search", slug: "depth-first-search" }
  - { name: "Breadth-First Search", slug: "breadth-first-search" }
  - { name: "Graph Theory", slug: "graph" }
  - { name: "Topological Sort", slug: "topological-sort" }
  - { name: "Directed Acyclic Graph", slug: "directed-acyclic-graph" }
sidebar:
  label: 'Course Schedule'
  badge: 'Medium'
---

### 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

```py
class Solution:
    def canFinish(self, numCourses: int, prerequisites: list[list[int]]) -> bool:
        indegree = [0] * numCourses
        adj = [[] for x in range(numCourses)]
        for prereq in prerequisites:
            adj[prereq[1]].append(prereq[0])
            indegree[prereq[0]] += 1

        queue = []

        for i in range(numCourses):
            if indegree[i] == 0:
                queue.append(i)

        visited = 0
        while queue:
            node = queue.pop(0)
            visited += 1
            for neighbor in adj[node]:
                indegree[neighbor] -= 1
                if indegree[neighbor] == 0:
                    queue.append(neighbor)

        return numCourses == visited
```