preprocess input graph

[HaagManuel]

• make graph undirected

• edge exists in one direction -> undirected edge of weight 1

• edge exists in both direction -> undirected edge of weight 2

• implementation idea: for each w in N(v), intersect N(v) and N(w)

[HaagManuel]

• returns adjacency list vec<vec<pair<node, weight>>>