Course Schedule

 

Course Schedule

There are a total of numCourses courses you have to take, labeled from 0 to numCourses-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?

 

Example 1:

Input: numCourses = 2, prerequisites = [[1,0]]
Output: true
Explanation: There are a total of 2 courses to take. 
             To take course 1 you should have finished course 0. So it is possible.

Example 2:

Input: numCourses = 2, prerequisites = [[1,0],[0,1]]
Output: false
Explanation: 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.

 

Constraints:

• The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
• You may assume that there are no duplicate edges in the input prerequisites.
• 1 <= numCourses <= 10^5

class Solution {

public:

    bool detectCyc(vector<int> adj[],int i,vector<int> &vis)

    {

        if(vis[i]==1)return true;

        if(vis[i]==2)return false;

        vis[i]=1;

        for(int k=0;k<adj[i].size();++k)

        {

            if(detectCyc(adj,adj[i][k],vis))

                return true;

        }

        vis[i]=2;

        return false;

    }

    bool canFinish(int num, vector<vector<int>>& pre) {

        if(pre.size()<=1)return true;

        vector<int> adj[num];

        for(int i=0;i<pre.size();++i)

            adj[pre[i][1]].push_back(pre[i][0]);

        

        vector<int> vis(num,0);

        

        for(int i=0;i<num;++i)

        {

            if(!vis[i])

                if(detectCyc(adj,i,vis))

                    return false;

        }

        return true;

            

    }

};