import java.util.ArrayList;

import java.util.HashMap;

import java.util.LinkedList;

import java.util.List;

import java.util.Map;

import java.util.Queue;

public class Prerequisites {

public boolean prerequisites(int n, int[][] prerequisites) {

Map<Integer, List<Integer>> graph = new HashMap<>();

int[] inDegrees = new int[n];

// Represent the graph as an adjacency list and record the in-

// degree of each course.

for (int[] edge : prerequisites) {

int prerequisite = edge[0];

int course = edge[1];

graph.putIfAbsent(prerequisite, new ArrayList<>());

graph.get(prerequisite).add(course);

inDegrees[course]++;

}

Queue<Integer> queue = new LinkedList<>();

// Add all courses with an in-degree of 0 to the queue.

for (int i = 0; i < n; i++) {

if (inDegrees[i] == 0) {

queue.offer(i);

}

}

int enrolledCourses = 0;

// Perform topological sort.

while (!queue.isEmpty()) {

int node = queue.poll();

enrolledCourses++;

if (graph.containsKey(node)) {

for (int neighbor : graph.get(node)) {

inDegrees[neighbor]--;

// If the in-degree of a neighboring course becomes 0, add

// it to the queue.

if (inDegrees[neighbor] == 0) {

queue.offer(neighbor);

}

}

}

}

// Return true if we've successfully enrolled in all courses.

return enrolledCourses == n;

}

}