A

```
require 'set'
##
# This class aims to represent weighted graphs
# (i.e. graphs for which edges between nodes have specific weights associated to them).
#
# Both directed (i.e. an edge between node U and node V does not imply an edge in the opposite direction)
# and undirected graphs are supported, depending on the constructor invocation.
class WeightedGraph
attr_reader :nodes
attr_reader :directed
def initialize(nodes: [], edges: {}, directed: true)
@nodes = Set[]
@edges = {}
@directed = directed
for node in nodes
add_node(node)
end
edges.each do |node, edges|
for neighbor, weight in edges
add_edge(node, neighbor, weight)
end
end
end
def add_node(node)
if include?(node)
raise ArgumentError, "node #{node} already exists in this graph!"
end
@nodes.add(node)
@edges[node] = {}
end
def add_edge(start_node, end_node, weight)
if has_neighbor?(start_node, end_node)
raise ArgumentError, "node #{start_node} already has an edge to #{end_node} in this graph!"
end
@edges[start_node][end_node] = weight
@edges[end_node][start_node] = weight unless directed
end
def edges(node)
unless include?(node)
raise ArgumentError, "node #{node} does not exist in this graph!"
end
@edges[node]
end
def empty?
nodes.empty?
end
def include?(node)
nodes.include?(node)
end
def has_neighbor?(start_node, end_node)
edges(start_node).key?(end_node)
end
def edge_weight(start_node, end_node)
edges(start_node)[end_node]
end
end
```