Obsidian Notes

Course Schedule II

3 min read

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 the ordering of courses you should take to finish all courses. If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array.

Example 1:

Input: numCourses = 2, prerequisites = 1,0
Output: [0,1]
Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1].

Example 2:

Input: numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]]
Output: [0,2,1,3]
Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0.
So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3].

Example 3:

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

Constraints:

  • 1 <= numCourses <= 2000
  • 0 <= prerequisites.length <= numCourses * (numCourses - 1)
  • prerequisites[i].length == 2
  • 0 <= ai, bi < numCourses
  • ai != bi
  • All the pairs [ai, bi] are distinct.

Approach

  • Same as 8. Course Schedule just we are adding result to a List
  • When using stream you need to provide mapping when using map as int
  • Topological sorting is an algorithm that arranges the vertices of a directed acyclic graph (DAG) in a way that respects the dependencies between them.
  • We use reverse to get that topological sort
class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        List<List<Integer>> graph = new ArrayList<>();
        for (int i = 0; i < numCourses; i++) {
            graph.add(new ArrayList<>());
        }
 
        for (int[] course: prerequisites) {
            graph.get(course[1]).add(course[0]);
        }
        
        int[] visited = new int[numCourses];
        List<Integer> result = new ArrayList<>();
        for (int i = 0; i < numCourses; i++) {
            if (hasCycle(graph, visited, result, i))
                return new int[0];
        }
 
        Collections.reverse(result);
 
        return result.stream().mapToInt(i -> i).toArray();
    }
 
    public boolean hasCycle(List<List<Integer>> graph, int[] visited, List<Integer> result, int course) {
        if (visited[course] == 1)
            return true;
        if (visited[course] == 2)
            return false;
 
        visited[course] = 1;
        for (int p: graph.get(course)) {
            if (hasCycle(graph, visited, result, p))
                return true;
        }
 
        visited[course] = 2;
        result.add(course);
        return false;        
    }
}

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