Course Schedule

Source

There are a total of n courses you have to take, labeled from 0 to n - 1.

Some courses may have prerequisites, for example to take course 0 you
have to first take course 1, which is expressed as a pair: [0,1]

Given the total number of courses and a list of prerequisite pairs, is
it possible for you to finish all courses?

For example:

2, [[1,0]]
There are a total of 2 courses to take. To take course 1 you should
have finished course 0. So it is possible.

2, [[1,0],[0,1]]
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.

Java

public class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        ArrayList[] graph = new ArrayList[numCourses];
        boolean[] visited = new boolean[numCourses];

        //create a 2D array
        for(int i=0; i<numCourses; i++){
            graph[i]= new ArrayList<Integer>();
        }

        for(int i=0; i<prerequisites.length; i++){
            graph[prerequisites[i][1]].add(prerequisites[i][0]);
        }

        for(int i=0; i<numCourses; i++){
            if(!dfs(graph, visited, i)){
                return false;
            }
        }
        return true;
    }

    public boolean dfs(ArrayList[] graph, boolean[] visited, int course){
        if(visited[course]) return false;
        visited[course]=true;

        for(int j=0; j<graph[course].size(); j++){
            if(!dfs(graph, visited, (int)graph[course].get(j))) return false;
        }
        visited[course]=false;
        return true;
    }
}