leetcode/Course Schedule at master · olivia-chen/leetcode · GitHub

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//DFS way
public class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        ArrayList<ArrayList<Integer>> adj = new ArrayList<ArrayList<Integer>>();
        boolean[] isExplored = new boolean[numCourses];
        for(int i = 0; i < numCourses; i++){
            adj.add(i, new ArrayList<Integer>());
        }
        for(int i = 0; i < prerequisites.length; i++){
            adj.get(prerequisites[i][0]).add(prerequisites[i][1]);
        }
        for(int i = 0; i < numCourses; i++){
            if(!DFS(i, isExplored, adj)){
               return false;
            }
        }
        return true;
    }
    private boolean DFS(int s, boolean[] isExplored, ArrayList<ArrayList<Integer>> adj){
        if(isExplored[s]){
            return false;
        }else{
            isExplored[s] = true;
        }
        for(int v : adj.get(s)){
            if(!DFS(v, isExplored, adj)){
                return false;
            }
        }
        isExplored[s] = false; /////////////////////// change to false, for next loop
        return true;
    }
}

//DFS Optimized
public class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        ArrayList<ArrayList<Integer>> adj = new ArrayList<ArrayList<Integer>>();
        boolean[] isExplored = new boolean[numCourses];
        for(int i = 0; i < numCourses; i++){
            adj.add(i, new ArrayList<Integer>());
        }
        for(int i = 0; i < prerequisites.length; i++){
            adj.get(prerequisites[i][0]).add(prerequisites[i][1]);
        }
        for(int i = 0; i < numCourses; i++){
            if(!DFS(i, isExplored, adj)){
               return false;
            }
        }
        return true;
    }
    private boolean DFS(int s, boolean[] isExplored, ArrayList<ArrayList<Integer>> adj){
        isExplored[s] = true;
        for(int v : adj.get(s)){
            if(isExplored[v]){
                return false;
            }else{
                if(!DFS(v, isExplored, adj)){
                    return false;
                }
            }
        }
        isExplored[s] = false;
        return true;
    }
}

//BFS way
public class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        ArrayList<ArrayList<Integer>> adj = new ArrayList<ArrayList<Integer>>();
        int[] incoming = new int[numCourses];
        int count = 0;
        Queue<Integer> frontier = new LinkedList<Integer>();
        for(int i = 0; i < numCourses; i++){
            adj.add(i, new ArrayList<Integer>());
        }
        for(int i = 0; i < prerequisites.length; i++){
            adj.get(prerequisites[i][0]).add(prerequisites[i][1]);
            incoming[prerequisites[i][1]]++;
        }
        for(int i = 0; i < incoming.length; i++){
            if(incoming[i] == 0){
                count++;
                frontier.add(i);
            }
        }
        while(!frontier.isEmpty()){
            int u = frontier.poll();
            for(int v : adj.get(u)){
                incoming[v]--;
                if(incoming[v] == 0){
                    frontier.add(v);
                    count++;
                }
            }
        }
        return count == numCourses;
    }
}