LeetCode-in-All

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.

Solution

object Solution {
    def canFinish(numCourses: Int, prerequisites: Array[Array[Int]]): Boolean = {
        import scala.collection.mutable.{Queue, ListBuffer}
        val indegree = Array.fill(numCourses)(0)
        val graph = Array.fill(numCourses)(new ListBuffer[Int])
        for (data <- prerequisites) {
            val course = data.head
            val prerequisiteCourse = data.last
            indegree(course) = indegree(course) + 1
            graph(prerequisiteCourse) += course
        }
        val startingCourses = indegree.zipWithIndex.filter(_._1.equals(0)).map(_._2)
        val queue = Queue[Int](startingCourses: _*)
        var courseTaken = 0
        while (queue.nonEmpty) {
            val current = queue.dequeue
            courseTaken = courseTaken + 1
            for (neighbor <- graph(current)) {
                indegree(neighbor) = indegree(neighbor) - 1
                if (indegree(neighbor).equals(0)) {
                    queue.enqueue(neighbor)
                }
            }
        }
        courseTaken == numCourses
    }
}

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