Solution: Parallel Courses

Statement

You are designing a course schedule for a university with n courses, labeled from 1 to n. The prerequisite requirements are given in an array, relations, where each $\text{relations}[i] = [\text{prevCourse}_i, \text{nextCourse}_i]$ means that $\text{prevCourse}_i$ must be completed before you can enroll in $\text{nextCourse}_i$.

You can take any number of courses during a single semester, as long as the prerequisite for each course has been met in a previous semester. ...