Task Dependency Scheduler

Problem Description

You are given a set of tasks, each identified by a unique string, and a list of dependency pairs. Each dependency pair (A, B) indicates that task A must be completed before task B can be started.

Your goal is to write a function or program that returns a valid order in which all tasks can be completed. If there are multiple valid orders, returning any one of them is acceptable. If no valid ordering exists (i.e., due to cyclic dependencies), your function should indicate that no valid schedule is possible.

Input

• A list of unique tasks (e.g., ["task1", "task2", "task3"]).
• A list of dependency pairs (e.g., [ ["task1", "task2"], ["task2", "task3"] ]), where each pair [A, B] means that A must be completed before B.

Output

• A valid ordering of tasks as a list, if one exists.
• Otherwise, an appropriate error message or indicator that a valid ordering cannot be determined.

Example

Given tasks:

["A", "B", "C", "D"]

and dependencies:

[["A", "B"], ["B", "C"], ["A", "C"], ["C", "D"]]

A possible valid ordering is:

["A", "B", "C", "D"]

Requirements

• Design your solution to handle a large number of tasks efficiently.
• Clearly handle and report cycles in the dependency graph.
• Provide well-documented code, and include any assumptions or limitations in your solution.

Good luck!