Topological Sort

There are numCourses courses labeled 0..numCourses-1 with a list of prerequisites ([a, b] meaning b must come before a). Return any valid ordering in which to take the courses, or an empty array if it's impossible (cycle exists). Solve with topological sort.

A tree is an undirected graph where any two vertices are connected by exactly one path. For a tree with n nodes labeled 0..n-1 and n-1 edges, you can pick any node as the root. The result is a rooted tree with some height. Return all root labels that minimize the resulting tree height (Minimum Height Trees, MHTs).

There is a new alien language whose alphabet you don't know. You are given a list of words sorted lexicographically by the rules of this alien language. Derive the order of letters in the alien language. If multiple valid orders exist, return any. If no valid order exists (the input is inconsistent), return "".