def bidirectional$\_$ucs$($graph$,$ start, goal$)$:

$"""$

Exercise $1$: Bidirectional Search.

Args:

graph $($ExplorableGraph$)$: Undirected graph to search.

start $($str$)$: Key for the start node.

goal $($str$)$: Key for the end node.

Returns:

The best path as a list from the start to the goal node $($including both$).$

$"""$

# TODO: finish this function!

raise NotImplementedError