Task Scheduler with Dependencies

Problem Description

You are given a list of tasks and a list of dependency pairs. Each dependency pair (U, V) indicates that task U must be completed before task V. Your goal is to write a function that returns a valid ordering of tasks that satisfies all the given dependencies.

Input

• A list of tasks (e.g., strings or integers).
• A list of dependency pairs where each pair is denoted as (U, V).

Output

• A list representing a valid order in which tasks can be completed. If no valid ordering exists because of cyclic dependencies, your function should indicate that it is impossible to complete all tasks.

Example

Given the tasks: ["A", "B", "C", "D"] and dependency pairs: [ ("A", "B"), ("B", "C"), ("A", "C"), ("C", "D") ], a possible valid ordering is ["A", "B", "C", "D"].

Requirements

• Ensure that your solution handles cases where multiple valid orderings exist.
• Handle edge cases where cyclic dependencies make it impossible to complete all tasks.

Implement your solution in the programming language of your choice.