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CourseSchedule.java
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CourseSchedule.java
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package Leetcode;
import java.util.*;
/**
* @author kalpak
*
* 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.
*
*/
public class CourseSchedule {
public static boolean canFinish(int numCourses, int[][] prerequisites) {
Map<Integer, List<Integer>> graph = new HashMap<>();
int[] indegree = new int[numCourses];
Deque<Integer> queue = new LinkedList<>();
// Initialize the graph
for(int i = 0; i < numCourses; i++) {
graph.put(i, new ArrayList<>());
}
// Put the edges into the graph.
// Put the edges ways as directed
// Update the in-degree
for(int[] edge : prerequisites) {
graph.get(edge[1]).add(edge[0]);
indegree[edge[0]]++;
}
// Add all nodes with indegree == 0
for(int i = 0; i < numCourses; i++) {
if(indegree[i] == 0)
queue.offer(i);
}
// Run BFS
while(!queue.isEmpty()) {
int current = queue.poll();
List<Integer> adjList = graph.get(current);
for(int i : adjList) {
indegree[i]--;
if(indegree[i] == 0)
queue.offer(i);
}
numCourses--;
}
return numCourses == 0;
}
public static void main(String[] args) {
int[][] prereq = new int[][]{{1,0},{0,1}};
System.out.println(canFinish(2, prereq));
}
}