Course Schedule

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Problem

There are numCourses courses labeled 0 to numCourses-1, and a list of prerequisite pairs where [a, b] means you must take course b before course a. Determine whether it is possible to finish all courses given these prerequisites.

Examples

Example 1

Input:numCourses = 2, prerequisites = [[1,0]]

Output:true

Example 2

Input:numCourses = 2, prerequisites = [[1,0],[0,1]]

Output:false

The two courses depend on each other.

Constraints

1 <= numCourses <= 2000

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How to approach it: the Graphs pattern

A graph models relationships as nodes and edges, which can be directed or undirected, weighted or not. Most graph problems come down to choosing the right traversal — DFS, BFS, or union-find — for what you're trying to find.

Look for this pattern when

The problem involves connections between items (islands, courses with prerequisites, friend networks, grids).
You need the shortest path in an unweighted graph (BFS) or need to explore every reachable node (DFS).
You're asking "are these two things connected?" repeatedly (union-find).

Read the full Graphs guide →

Video walkthroughs

Original problem on LeetCode ↗