Clone a Graph

graph maps string IDs to lists of neighbor IDs; all listed neighbors exist. It may contain cycles, self-loops, repeated neighbors, or disconnected components. Return a deep independent adjacency dictionary for only the nodes reachable from start. Preserve each node's exact neighbor-list order and duplicates; dictionary key order is irrelevant. Return {} if start is None or absent. Build new neighbor lists rather than returning aliases into graph. For this JSON-based exercise, a clone is represented by its reachable adjacency lists, not custom node objects. Examples (JSON; null means Python None): Input: {"graph": {"a": ["b"], "b": ["a"], "c": []}, "start": "a"} Output: {"a": ["b"], "b": ["a"]} Input: {"graph": {"a": ["a", "a"]}, "start": "a"} Output: {"a": ["a", "a"]}

def clone_graph(graph: dict[str, list[str]], start: str | None) -> dict[str, list[str]]:
    """Copy the reachable graph while preserving edges and neighbor order."""
    pass