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If given that all edges in a graph $G$ are of equal weight $c$, can one use breadth-first search (BFS) in order to produce a minimal spanning tree in linear time?

Intuitively this sounds correct, as BFS does not visit a node twice, and it only traverses from vertex $v$ to vertex $u$ iff it hasn't visited $u$ before, such that there aren't going to be any cycles, and if $G$ is connected it will eventually visit all nodes. Since the weight of all edges is equal, it doesn't matter which edges the BFS chose.

Does my reasoning make any sense?

Juho
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TheNotMe
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3 Answers3

15

If your graph is unweighted, or equivalently, all edges have the same weight, then any spanning tree is a minimum spanning tree. As you observed, you can use a BFS (or even DFS) to find such a tree in time linear in the number of edges.

David Richerby
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Juho
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3

If all edge costs are equal, then any spanning tree is also a minimum spanning tree. In this case, any algorithm that solves REACHABILITY solves MST as well. 

Let S = {v0} be a set of nodes initially containing v0
Mark v0
Parent[v0] = -1
While S is not empty
  Remove a vertex v from S
  For all edges (v,u)
    If u is unmarked
      Mark it and add it to S
      Parent[u] = v

You can recover the tree from the Parent relation. If S.Remove and S.Add take constant time, then the algorithm takes $\cal O(v+e)=\cal O(v^2)$ where $v,e$ are the number of vertices and edges.

mrk
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-3

If all the edges are of equal weight we can use:

-BFS -DFS -Dijkstra's algorithm -Prim's algorithm

But you cannot use

-kruskal's algorithm

Nandkishor Nangre
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