Task Scheduler

You are given a list of unique tasks represented as strings and a list of dependency pairs where each pair [A, B] indicates that task A must be completed before task B can begin.

Write a function or method that takes in two inputs:

1. tasks: an array of strings representing the tasks.
2. dependencies: an array of pairs (2-element arrays) where each pair [A, B] enforces that task A must come before task B.

Your function should output a valid ordering of tasks such that all dependency requirements are satisfied. If no such order is possible (i.e., there exists a cycle in the dependencies), your function should detect this condition and indicate that the scheduling is impossible.

Your solution should consider edge cases and be time efficient. The implementation language is up to you.

Example:

Input:

tasks = ["a", "b", "c", "d"]
dependencies = [["a", "b"], ["b", "c"], ["a", "c"], ["c", "d"]]

A valid output ordering could be: ["a", "b", "c", "d"]

If dependencies instead form a cycle such as:

tasks = ["a", "b", "c"]
dependencies = [["a", "b"], ["b", "c"], ["c", "a"]]

then the function should indicate that no valid scheduling order exists.