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:
- 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.
class Solution { public: bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) { vector<vector<int>> graph(numCourses, vector<int>(0)); vector<int> in(numCourses, 0); for (auto a : prerequisites) { graph[a.second].push_back(a.first); ++in[a.first]; } queue<int> 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; } };