Return valid task order from prerequisites

You are given **N** tasks labeled `0..N-1` and a list of directed dependency pairs `[(a,b), ...]` meaning **task `a` must be done before task `b`**. 1) Return **one valid execution order** of all tasks that satisfies all dependencies. 2) If it is **impossible** (the graph has a cycle), return an empty list. Follow-ups that may be asked: - If multiple valid orders exist, return the **lexicographically smallest** order. - Return **all** valid topological orders (if feasible for the input size). - State the **time and space complexity** of your approach. Constraints can be assumed typical for interview settings (e.g., up to 1e5 nodes/edges for part 1; much smaller for “all orders”).