Given n courses and prerequisite relations, return the minimum number of semesters needed to complete all courses. If it is impossible, return -1. Pattern focus: Indegree. Grouping nodes by indegree-zero layers gives the semester count naturally.
n = number of courses, relations = prerequisite pairs [u, v]
minimum semesters needed, or -1 if impossible
Example 1:
Input:
n = 3 relations = [[1,3],[2,3]]
Output:
2
Explanation:
Courses 1 and 2 can be taken together, then course 3.
Example 2:
Input:
n = 3 relations = [[1,2],[2,3],[3,1]]
Output:
-1
Explanation:
A cycle makes completion impossible.
Example 3:
Input:
n = 4 relations = [[1,2],[2,3],[3,4]]
Output:
4
Explanation:
The chain forces one course per semester.