Course Schedule
Desicription
There are a total of n courses you have to take, labeled from 0
to n1
.
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:
1 2 3 4
 Input: 2, [[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:
1 2 3 4 5
 Input: 2, [[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.

Note:
 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.
Solution
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 class Solution { public: bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) { vector<unordered_set<int>> graph(numCourses); for(auto it : prerequisites) { graph[it.second].insert(it.first); } vector<int> degrees(numCourses, 0); for(auto it : graph) { for(auto index : it) { degrees[index]++; } } for(int i = 0; i < numCourses; i++) { int index = 0; for(; index < numCourses; index++) { if(degrees[index] == 0) { break; } } if(index == numCourses) { return false; } degrees[index] = 1; for(auto it : graph[index]) { degrees[it]; } } return true; } };
