Approximates a minimum Steiner tree by greedily connecting terminals with the shortest edges.

# Steiner Tree - Greedy Approach (Not Optimal)
# Description: Approximates a minimum Steiner tree by greedily connecting terminals with the shortest edges.

def steiner_tree(graph, terminals):
    """
    Approximates the Steiner Tree using a greedy approach.

    This algorithm connects terminal nodes by repeatedly adding the shortest 
    edge that connects an existing tree node to a new node.

    Args:
        graph (dict): Adjacency list representation with edge weights.
        terminals (list): Nodes that must be included in the final tree.

    Returns:
        set: Nodes in the approximated Steiner tree.

    Time Complexity: O(V^2), where V is the number of vertices
    Space Complexity: O(V)

    Note: This is a greedy approximation and may not find the optimal Steiner tree.
    """
    # Initialize the tree with terminal nodes
    tree = set(terminals)

    # Iteratively expand the tree by adding the shortest connecting edge
    while len(tree) < len(graph):
        # Find the shortest edge connecting the current tree to a new node
        shortest_edge = None
        min_distance = float('inf')

        # Check edges from each node in the current tree
        for current_node in tree:
            for neighbor, distance in graph[current_node].items():
                # Find the shortest edge to a node not yet in the tree
                if neighbor not in tree and distance < min_distance:
                    shortest_edge = (current_node, neighbor)
                    min_distance = distance

        # If no connecting edge is found, break the loop
        if shortest_edge is None:
            break

        # Add the new node to the tree
        tree.add(shortest_edge[1])

    return tree