Contraction Undirected multigraph GV,E) 。multigraph: ·allow parallel edges ·but no self-loop contract(e):e uv is an edge merges two endpoints u and vContraction • Undirected multigraph • multigraph: • allow parallel edges • but no self-loop • contract( ): is an edge • merges two endpoints and G(V, E) e e = uv u v