Course Schedule
Source
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.
Java
public class Solution {
public boolean canFinish(int numCourses, int[][] prerequisites) {
ArrayList[] graph = new ArrayList[numCourses];
boolean[] visited = new boolean[numCourses];
for(int i=0; i<numCourses; i++){
graph[i]= new ArrayList<Integer>();
}
for(int i=0; i<prerequisites.length; i++){
graph[prerequisites[i][1]].add(prerequisites[i][0]);
}
for(int i=0; i<numCourses; i++){
if(!dfs(graph, visited, i)){
return false;
}
}
return true;
}
public boolean dfs(ArrayList[] graph, boolean[] visited, int course){
if(visited[course]) return false;
visited[course]=true;
for(int j=0; j<graph[course].size(); j++){
if(!dfs(graph, visited, (int)graph[course].get(j))) return false;
}
visited[course]=false;
return true;
}
}