DFS is determined whether or not there has circumferential FIG.

DFS is determined whether or not there has circumferential FIG.

There are three cases to a node, yet access is available, end of the visit
we use 0, 1, -1, is available for representation still not out of recursion, if nodes are connected to access
instructions on accessing the course of a road in DFS the same node twice, indicating that the ring has
the following implemented in code

class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
            if(findOrder(numCourses,prerequisites))
            {
                return false;
            }
            else
            {
                return true;
            }
    }
    int[] state ;
    Map<Integer,List<Integer>> map =new HashMap<Integer,List<Integer>>();
    void init(int numCourses, int[][] prerequisites)//初始化操作
    {
        state = new int[numCourses];//存放状态
        for(int i=0;i<numCourses;i++)
        {
            map.put(i,new ArrayList<Integer>());
        }
        for(int i=0;i<prerequisites.length;i++)
        {
            map.get(prerequisites[i][1]).add(prerequisites[i][0]);//建立领接列表存储节点
        }
    }
    public boolean findOrder(int numCourses, int[][] prerequisites) {
        init(numCourses,prerequisites);
        for(int i=0;i<numCourses;i++)
            {
                if(DFS(i))
                    return true;
                for(int k=0;k<numCourses;k++)
                {
                    state[k]=0;
                }
            }
            return false;
    }
     boolean DFS(int start)
  {
      if(state[start]==-1)//防止出现死循环
      {
            return  false;
      }
     if(state[start]==1)//正在访问
     {
         return true;
     }
     else
     {
         state[start]=1;
         for(int i=0;i<map.get(start).size();i++)
         {
             if(DFS(map.get(start).get(i)))//发现有环，一直往上回退直到退出
                 return true;
        }
         state[start]=-1;
     }
     return  false;
  }
}