LeetCode – Course Schedule (Java)
There are a total of n courses you have to take, labeled from 0 to n  1. Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair: [0,1]. Given the total number of courses and a list of prerequisite pairs, is it possible for you to finish all courses?
For example, given 2 and [[1,0]], there are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible.
For another example, given 2 and [[1,0],[0,1]], 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.
Analysis
This problem can be converted to finding if a graph contains a cycle.
Java Solution 1  BFS
This solution uses breathfirst search and it is easy to understand.
public boolean canFinish(int numCourses, int[][] prerequisites) { if(prerequisites == null){ throw new IllegalArgumentException("illegal prerequisites array"); } int len = prerequisites.length; if(numCourses == 0  len == 0){ return true; } // counter for number of prerequisites int[] pCounter = new int[numCourses]; for(int i=0; i<len; i++){ pCounter[prerequisites[i][0]]++; } //store courses that have no prerequisites LinkedList<Integer> queue = new LinkedList<Integer>(); for(int i=0; i<numCourses; i++){ if(pCounter[i]==0){ queue.add(i); } } // number of courses that have no prerequisites int numNoPre = queue.size(); while(!queue.isEmpty()){ int top = queue.remove(); for(int i=0; i<len; i++){ // if a course's prerequisite can be satisfied by a course in queue if(prerequisites[i][1]==top){ pCounter[prerequisites[i][0]]; if(pCounter[prerequisites[i][0]]==0){ numNoPre++; queue.add(prerequisites[i][0]); } } } } return numNoPre == numCourses; } 
Java Solution 2  DFS
public boolean canFinish(int numCourses, int[][] prerequisites) { if(prerequisites == null){ throw new IllegalArgumentException("illegal prerequisites array"); } int len = prerequisites.length; if(numCourses == 0  len == 0){ return true; } //track visited courses int[] visit = new int[numCourses]; // use the map to store what courses depend on a course HashMap<Integer,ArrayList<Integer>> map = new HashMap<Integer,ArrayList<Integer>>(); for(int[] a: prerequisites){ if(map.containsKey(a[1])){ map.get(a[1]).add(a[0]); }else{ ArrayList<Integer> l = new ArrayList<Integer>(); l.add(a[0]); map.put(a[1], l); } } for(int i=0; i<numCourses; i++){ if(!canFinishDFS(map, visit, i)) return false; } return true; } private boolean canFinishDFS(HashMap<Integer,ArrayList<Integer>> map, int[] visit, int i){ if(visit[i]==1) return false; if(visit[i]==1) return true; visit[i]=1; if(map.containsKey(i)){ for(int j: map.get(i)){ if(!canFinishDFS(map, visit, j)) return false; } } visit[i]=1; return true; } 
<pre><code> String foo = "bar"; </code></pre>

Ankit Shah

CRH

Jun

ryanlr