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?
Example 1:
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:
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.
考察:图,拓扑排序;
class Solution {
public:
bool canFinish(int numCourses, vector<vector<int>>& prerequisites){
vector<vector<int>> graph(numCourses, vector<int>(0));
vector<int> in(numCourses, 0);
for (int i = 0; i < prerequisites.size(); i ++) {
graph[prerequisites[i][1]].push_back(prerequisites[i][0]);
in[prerequisites[i][0]]++;
}
queue<int> que;
for (int i = 0; i < numCourses; i ++) {
if (in[i] == 0)
que.push(i);
}
while(!que.empty()) {
int cur = que.front();
que.pop();
for (int j = 0; j < graph[cur].size(); j ++) {
in[graph[cur][j]] --;
if (in[graph[cur][j]] == 0)
que.push(graph[cur][j]);
}
}
for (int i = 0; i < numCourses; i ++) {
if (in[i] != 0)
return false;
}
return true;
}
};