Implement ordering with dependency constraints

You are given n items labeled 0..n−1 and a list of precedence constraints edges[i] = [u, v] meaning u must appear before v. Return a valid ordering of all items that satisfies all constraints. If multiple orders exist, return the lexicographically smallest by item ID; if a cycle exists, return "IMPOSSIBLE." Analyze time and space complexity and provide working code. Follow-up: extend your solution to support groups where certain items must appear as contiguous blocks; respect both intra-group and inter-group dependencies and return any valid sequence or "IMPOSSIBLE."