课程表问题

Posted 2022-12-01 Alice_yufeng

class Solution 
    List<List<Integer>> edges;
    int[] visited;
    boolean valid = true;

    public boolean canFinish(int numCourses, int[][] prerequisites) 
        edges = new ArrayList<List<Integer>>();
        for (int i = 0; i < numCourses; ++i) 
            edges.add(new ArrayList<Integer>());
        
        visited = new int[numCourses];
        for (int[] info : prerequisites) 
            edges.get(info[1]).add(info[0]);
        
        for (int i = 0; i < numCourses && valid; ++i) 
            if (visited[i] == 0) 
                dfs(i);
            
        
        return valid;
    

    public void dfs(int u) 
        visited[u] = 1;
        for (int v: edges.get(u)) 
            if (visited[v] == 0) 
                dfs(v);
                if (!valid) 
                    return;
                
             else if (visited[v] == 1) 
                valid = false;
                return;
            
        
        visited[u] = 2;