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, return the ordering of courses you should take to finish all courses.
There may be multiple correct orders, you just need to return one of them. If it is impossible to finish all courses, return an empty array.
For example:
2, [[1,0]]
There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1]
4, [[1,0],[2,0],[3,1],[3,2]]
There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0. So one correct course order is [0,1,2,3]. Another correct ordering is[0,2,1,3].
解题思路：
参考上题，Java for LeetCode 207 Course Schedule【Medium】
修改下代码即可，JAVA实现如下：
public int[] findOrder(int numCourses, int[][] prerequisites) {
int[] res=new int[numCourses];
List<Set<Integer>> posts = new ArrayList<Set<Integer>>();
for (int i = 0; i < numCourses; i++)
posts.add(new HashSet<Integer>());
for (int i = 0; i < prerequisites.length; i++)
posts.get(prerequisites[i][1]).add(prerequisites[i][0]);
// count the pre-courses
int[] preNums = new int[numCourses];
for (int i = 0; i < numCourses; i++) {
Set<Integer> set = posts.get(i);
Iterator<Integer> it = set.iterator();
while (it.hasNext()) {
preNums[it.next()]++;
}
}
// remove a non-pre course each time
for (int i = 0; i < numCourses; i++) {
// find a non-pre course
int j = 0;
for ( ; j < numCourses; j++) {
if (preNums[j] == 0) break;
}
// if not find a non-pre course
if (j == numCourses) return new int[0];
res[i]=j;
preNums[j] = -1;
// decrease courses that post the course
Set<Integer> set = posts.get(j);
Iterator<Integer> it = set.iterator();
while (it.hasNext()) {
preNums[it.next()]--;
}
}
return res;
}