[leetcode#210] Course Schedule II

problem:

There is a total of n courses you have to take, labeled from 0 to n - 1 .

Some courses May has 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, return the ordering of courses you should take to fini SH all courses.

There may is multiple correct orders, you just need to return one of the them. If It is impossible-to-finish all courses, return an empty array.

For example:

2, [[1,0]]

There is a total of 2 courses to take. To take course 1 should has finished course 0. So the correct course order is[0,1]

4, [[1,0],[2,0],[3,1],[3,2]]

There is a total of 4 courses to take. To take course 3 should has finished both courses 1 and 2. Both Courses 1 and 2 should is taken after you finished course 0. So one correct course order is [0,1,2,3] . Another correct ordering is [0,2,1,3] .

Note:
The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more on how a graph is represented.

Analysis:

This problem was an updated version of "Course Schedule". The only extra thing we need to Dois -use-a array to record the order those courses were unlocked.        Once a element is poped out of the queue, we add to our result array.  while(!Queue.isempty ()) {            intCur =Queue.poll (); Visited[cur]=true; Ret[count]=cur; Count++; ...        } But Don' t be happy too earily!!! For course Schedule one we have writtenintLen =prerequisites.length;if(len = = 0)    return true; for Thisproblem, I had mistakely update it into:intLen =prerequisites.length;if(len = = 0)    return New int[0]; Which causes the Error:input:1, []output:[]expected:special judge:no expected Output available. Actually, all numcourses is inherently unlocked, and the programm logic could handle This  Case: for(inti = 0; i < numcourses; i++) {    //All Pre counter are 0    if(Pre_counter[i] = = 0) {queue.offer (i); }}then while(!Queue.isempty ()) {    intCur =Queue.poll (); Visited[cur]=true; Ret[count]=cur; ...}

Solution:

 Public classSolution { Public int[] FindOrder (intNumcourses,int[] Prerequisites) {        if(Prerequisites = =NULL)            Throw NewIllegalArgumentException ("The prerequisites matrix is not valid"); intLen =prerequisites.length; Boolean[] visited =New Boolean[numcourses]; int[] ret =New int[numcourses]; int[] Pre_counter =New int[numcourses]; intCount = 0; Queue<Integer> queue =NewLinkedlist<integer> ();  for(inti = 0; i < Len; i++) {pre_counter[prerequisites[i][0]]++; }         for(inti = 0; i < numcourses; i++) {            if(Pre_counter[i] = = 0) {queue.offer (i); }        }         while(!Queue.isempty ()) {            intCur =Queue.poll (); Visited[cur]=true; Ret[count]=cur; Count++;  for(inti = 0; i < Len; i++) {                if(Prerequisites[i][1] = =cur) {pre_counter[prerequisites[i][0]]--; if(pre_counter[prerequisites[i][0] [= 0 && visited[prerequisites[i][0]] = =false) Queue.offer (prerequisites[i][0]); }            }        }        if(Count = =numcourses)returnret; Else            return New int[0]; }}

[leetcode#210] Course Schedule II