There are a total of n courses you have to take labelled from 0 to n - 1.
Some courses may have prerequisites, for example, if prerequisites[i] = [ai, bi] this means you must take the course bi before the course ai.
Given the total number of courses numCourses and a list of the prerequisite pairs, return the ordering of courses you should take to finish all courses.
If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array.
Example 1:
Input: numCourses = 2, prerequisites = [[1,0]]
Output: [0,1]
Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1].
Example 2:
Input: numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]]
Output: [0,2,1,3]
Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0.
So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3].
Example 3:
Input: numCourses = 1, prerequisites = []
Output: [0]
package leetcode;
import java.util.*;
/**
* Created by mkhwang on 2021/05/01
* Email : orange2652@gmail.com
* Github : https://github.com/myeongkwonhwang
*/
public class CourseScheduleII {
public int[] findOrder(int numCourses, int[][] prerequisites) {
Map<Integer, List<Integer>> adjList = new HashMap<>();
int[] indegree = new int[numCourses];
int[] topologicalOrder = new int[numCourses];
for (int i = 0; i < prerequisites.length; i++) {
int dest = prerequisites[i][0];
int src = prerequisites[i][1];
List<Integer> lst = adjList.getOrDefault(src, new ArrayList<>());
lst.add(dest);
adjList.put(src, lst);
indegree[dest] += 1;
}
Queue<Integer> q = new LinkedList<>();
for (int i = 0; i < numCourses; i++) {
if (indegree[i] == 0) {
q.add(i);
}
}
int i = 0;
while (!q.isEmpty()) {
int node = q.remove();
topologicalOrder[i++] = node;
if (adjList.containsKey(node)) {
for (Integer neighbor : adjList.get(node)) {
indegree[neighbor]--;
if (indegree[neighbor] == 0) {
q.add(neighbor);
}
}
}
}
if (i == numCourses) {
return topologicalOrder;
}
return new int[0];
}
}