LeetCode刷题（C++）——Course Schedule

待我称王封你为后i 2022-06-10 10:24 147阅读 0赞

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 [how a graph is represented][].
2.  You may assume that there are no duplicate edges in the input prerequisites.

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

[how a graph is represented]: https://www.khanacademy.org/computing/computer-science/algorithms/graph-representation/a/representing-graphs