1+ package solutions ;
2+ 3+ import java .util .*;
4+ 5+ // [Problem] https://leetcode.com/problems/course-schedule
6+ class CourseSchedule {
7+ // Topological sort
8+ // O(v + e) time, O(v + e) space
9+ // where v = number of courses, e = number of prerequisites
10+ public boolean canFinish (int numCourses , int [][] prerequisites ) {
11+ Map <Integer , List <Integer >> nextCourseMap = new HashMap <>();
12+ int [] numPrerequisites = new int [numCourses ];
13+ for (int [] courseToPrerequisite : prerequisites ) {
14+ int course = courseToPrerequisite [0 ];
15+ int prerequisite = courseToPrerequisite [1 ];
16+ nextCourseMap .putIfAbsent (prerequisite , new ArrayList <>());
17+ nextCourseMap .get (prerequisite ).add (course );
18+ numPrerequisites [course ]++;
19+ }
20+ Queue <Integer > coursesToTake = new LinkedList <>();
21+ for (int i = 0 ; i < numCourses ; i ++) {
22+ if (numPrerequisites [i ] == 0 ) {
23+ coursesToTake .add (i );
24+ }
25+ }
26+ int count = 0 ;
27+ while (!coursesToTake .isEmpty ()) {
28+ int currentCourse = coursesToTake .poll ();
29+ List <Integer > nextCourses = nextCourseMap .get (currentCourse );
30+ if (nextCourses != null ) {
31+ for (int nextCourse : nextCourses ) {
32+ if (--numPrerequisites [nextCourse ] == 0 ) {
33+ coursesToTake .add (nextCourse );
34+ }
35+ }
36+ }
37+ count ++;
38+ }
39+ return count == numCourses ;
40+ }
41+ 42+ // Test
43+ public static void main (String [] args ) {
44+ CourseSchedule solution = new CourseSchedule ();
45+ 46+ int numCourses1 = 2 ;
47+ int [][] prerequisites1 = {{1 , 0 }};
48+ boolean expectedOutput1 = true ;
49+ boolean actualOutput1 = solution .canFinish (numCourses1 , prerequisites1 );
50+ System .out .println ("Test 1 passed? " + (expectedOutput1 == actualOutput1 ));
51+ 52+ int numCourses2 = 3 ;
53+ int [][] prerequisites2 = {{1 , 0 }, {0 , 1 }};
54+ boolean expectedOutput2 = false ;
55+ boolean actualOutput2 = solution .canFinish (numCourses2 , prerequisites2 );
56+ System .out .println ("Test 2 passed? " + (expectedOutput2 == actualOutput2 ));
57+ }
58+ }
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