Course Schedule

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.

查看拓扑排序是否有环

class Solution {
public:
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        queue<int>q;
        int cnt = 0;
        int indep[numCourses+1] = {0};
        for(int i = 0; i < prerequisites.size(); i++)
        {
            indep[prerequisites[i].first]++;
        }
        for(int i = 0; i < numCourses; i++)
        {
            if(indep[i] == 0)
            {
                q.push(i);
            }
        }
        while(!q.empty())
        {
            int head = q.front();
            q.pop();
            cnt++;
            for(int i = 0; i < prerequisites.size(); i++)
            {
                if(prerequisites[i].second == head) {
                    indep[prerequisites[i].first]--;
                    if(indep[prerequisites[i].first] == 0)
                        q.push(prerequisites[i].first);
                }
            }
        }
        return cnt==numCourses;
    }
};