There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai.
For example, the pair [0, 1], indicates that to take course 0 you have to first take course 1.Return true if you can finish all courses. Otherwise, return false.
Example 1: Input: numCourses = 2, prerequisites = [[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: numCourses = 2, prerequisites = [[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.
Constraints:
1 <= numCourses <= 105 0 <= prerequisites.length <= 5000 prerequisites[i].length == 2 0 <= ai, bi < numCourses All the pairs prerequisites[i] are unique.
- code
class Solution {
public boolean canFinish(int numCourses, int[][] prerequisites) {
HashMap<Integer, ArrayList<Integer>> adja = new HashMap();
int[] indegree = new int[numCourses];
for(int[] edge: prerequisites){
int start = edge[1];
int end = edge[0];
indegree[end] ++;
if (adja.containsKey(start)){
adja.get(start).add(end);
}else{
ArrayList<Integer> nei = new ArrayList();
nei.add(end);
adja.put(start, nei);
}
}
LinkedList<Integer> q = new LinkedList();
for (int i=0; i < numCourses; ++i){
if (indegree[i] == 0){
q.offer(i); // add to last
}
}
int added = 0;
while(!q.isEmpty()){
int cur = q.poll(); // pop first
if (++added == numCourses){
return true;
}
if (adja.containsKey(cur)){
for (int nei: adja.get(cur)){
if (--indegree[nei] == 0){
q.offer(nei);
}
}
}
}
return false;
}
}