Connected Components (Undirected)

Portions of a Graph are said to be connected when there is no way to separate the vertices without edges crossing between the parts. A common task is to identify all connected components of a graph. Consider the graph below, it is made up of three connected components.

connected components

This algorithm identifies the connected components in a graph by marking each vertex with a component id. Any vertex with a matching id is part of the same connected component. Differing ids indicate no connectivity. Although shown below with BFS, DFS works equally well for this purpose.

Applications

  • Connectivity - In a network, ensure that every node is reachable.

Asymptotic Complexity

Pseudo Code

G = input graph
q = queue data structure
side effects: marks all vertices with a component id

component_id = 0

for each vertex (v) in G:
    if v is NOT conquered:
        component_id = component_id + 1
        conquer(v)
        v->component_id = component_id
        q->enqueue(v)

        while q is not empty:
            v = q->dequeue
            for each edge in v:
                w = edge->head
                if w is NOT conquered:
                    conquer(w)
                    w->component_id = component_id
                    q->enqueue(w)

Source Code

Full Repo

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