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Given an adjacency-list representation of a multi graph $G=(V,E)$, describe an $O(V+E)$ time algorithm to compute the adjacency-list representation of the “equivalent” undirected graph $G’=(V,E’)$ , where $E’$ is consists of the edges in $E$ with all multiple edges between two vertices replaced by a single edge and with all self-loops removed.
in Algorithms by Boss (42.7k points) | 38 views

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Create a new adjacency-list Adj of size |V| and an empty matrix M of size |V|^2  first. For each vertex u in the multigraph G, we iterably examine each vertex v of Adj[u].

•If M(u, v) == ∅, mark M(u, v) true, then add v to Adj[u]

•If M(u, v) == true, do nothing.

Since we lookup in the adjacency-list Adj for |V + E| times, the time complexity is O(V + E).


    let Adj'[1..|V|] be a new adjacency-list
    let M be |V| × |V|
    for each vertex u ∈ G.V
        for each v ∈ Adj[u]
            if M[u, v] == Ø && u != v
                M[u, v] = true
                INSERT(Adj'[u], v)
by (11 points)

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