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

For example:

2, [[1,0]]

There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible.

2, [[1,0],[0,1]]

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.

Note:

1. The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about .
2. You may assume that there are no duplicate edges in the input prerequisites.

class Solution {public:    bool canFinish(int numCourses, vector
     
      
       >& prerequisites) {        vector
       
        
         > graph(numCourses, vector
         
          (0));         vector
          
            in(numCourses, 0); for (auto a : prerequisites) { graph[a.second].push_back(a.first); ++in[a.first]; } queue
           
             q; for (int i = 0; i < numCourses; ++i) { if (in[i] == 0) q.push(i); } while (!q.empty()) { int t = q.front(); q.pop(); for (auto a : graph[t]) { --in[a]; if (in[a] == 0) q.push(a); } } for (int i = 0; i < numCourses; ++i) { if (in[i] != 0) return false; } return true; }};