Course Schedule

class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        ArrayList<ArrayList<Integer>> graph = new ArrayList<>();
        for(int i=0;i<numCourses;i++){
            graph.add(new ArrayList<>());
        }
        
        for(int i=0;i<prerequisites.length;i++){
            int u=prerequisites[i][0];
            int v=prerequisites[i][1];
            graph.get(v).add(u);
        }
        
        
        
        //detect a cycle dfs
        
        
        boolean[] recStack = new boolean[numCourses];
        boolean[] visited = new boolean[numCourses];
        
        for(int i=0;i<graph.size();i++){
            if(visited[i]==false){
            if(dfs(i,graph,visited,recStack)){
                return false;
            }
            }
        }
        return true;
        
    }
    
    private boolean dfs(int currentVertex,ArrayList<ArrayList<Integer>> graph, boolean[] visited, boolean[] recStack){
     if(visited[currentVertex]==false){
         visited[currentVertex]=true;
         recStack[currentVertex]=true;
         for(int neighbour:graph.get(currentVertex)){
             if(visited[neighbour]==false&&dfs(neighbour,graph,visited,recStack)){
                 return true;
             } else if(recStack[neighbour]==true){
                 return true;
             }
         }
         recStack[currentVertex]=false;
     }
        return false;
    }
}