Lecture 12
Minimum Spanning Tree
Motivating Example: Point to
Multipoint Communication
• Single source, Multiple Destinations
• Broadcast
–
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All nodes in the network are destinations
• Multicast
– Some nodes in the network are destinations
• Only one copy of the information travels along
common edges
Message replication along forking points only.
Spanning Tree
• We consider undirected graphs here.
• A tree is a connected graph without a cycle
• A spanning tree is a tree which has all
vertices of the graph
• There may be multiple spanning trees
• We need to choose the minimum weight
tree for broadcast.
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Blue edge spanning tree is
the minimum weight
spanning tree
Properties of a Tree
• A tree of V vertices has V-1 edges
• There exists a unique path between any two
vertices of a tree.
• Adding any edge to a tree creates a unique
cycle. Breaking any edge on this cycle
restores a tree.
Minimum Spanning Tree
Construction
• We maintain a set of edges A, which is initially
empty.
• Edges are added to A one at a time such that
finally A becomes a minimum spanning tree.
• Edges are never removed from A.
• So ``safe’’ edges must be added to A, i.e., at any
stage A must be a part of a spanning tree.
Safe Edge Addition
• Consider a cut in a graph (a cut consists of 2 sets which
partition the vertex set).
A
B
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3
E 1
F
{A,B,C}
{D,E,F}
constitute a cut.
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C
D
•A cut respects a set of edges I if no edge in the set crosses the cut.
Let I = {edge AB, edge EF}.Cut
{A,B,C}, {D,E,F} respects set I
•A minimum weight edge crossing a cut is denoted a light
weight edge in the cut
Edge CD is a light weight edge in
• Let A be a subset of a MST (minimum
weight spanning tree).
• Let (S, V – S) be any cut that respects A
• Let edge (u, v) be a light edge crossing the
cut
• Then A  (u, v) is subset of a MST
Assume that all the edge weights are distinct.
Let no MST containing A contain edge (u, v).
Consider a MST T containing A
Add the edge (u, v) to T
Since (u, v) is not in T, T 
(u, v) contains a
cycle, and (u, v) is in the cycle.
Edge (u, v) are in the opposite sides of the cut.
Since any cycle must cross the cut even number
of times, there exists at least one other edge (x, y)
crossing the cut. Clearly, w(x, y) > w(u, v).
The edge (x, y) is not in A because (x, y) crosses the
cut, and the cut respects A.
Removing (x, y) from the cycle, breaks the cycle and hence
creates a spanning tree, T’, s.t. T’ = T 
(u, v) – (x, y)
w(T’) = w(T) + w(u, v) – w(x, y)
 w(T)
(as w(u, v) < w(x, y))
This contradicts the fact that T is a MST.
• So we always find a cut that respects A,
• And add a light edge across the cut to A.
• Kruskals and Prims algorithms find the cut
differently.
Kruskals Algorithm
•
1.
2.
3.
A=
For each vertex u in V, Create_Set(u)
Sort E in increasing order by weight w
For each edge (u,v) in the sorted list
–
If Set(u)= Set(v)
•
•
•
Add (u,v) to A
Union Set(u) and Set(v)
Return A;
Complexity Analysis
• The operations create set, testing whether set(u)
== set(v), union operations can be done in log V
operations, using Union find data structure.
• Step 1 can be done in Vlog V
• Step 2 can be done in Elog E
• Step 3 can be done in Elog V
• Overall complexity is O(Vlog V + Elog E + Elog
V) or O((V + E)log V)
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Prims Algorithm
• Maintains a set of vertices S already in the
spanning tree.
• Initially, S consists of one vertex r, selected
arbitrarily.
• For every vertex u in V – S, maintain the weight
of the lightest edge between u and any vertex in S.
• If there is no edge between u and S, then this
weight associated with u is infinity.
• Add the vertex with the least weight to S.
• This is in effect adding the light weight
edge crossing the cut S and V-S.
• Whenever a vertex is added to S, the
weights of all its neighbors are reduced, if
necessary.
Pseudo-Code
S=
For each u in V, key[u] = 
Key[r] = 0
Pred[r] = NULL
While V = S
u = Extract_Min(V-S)
For each (v in Adj(u))
if (v not in S) key(v) = min(w(u, v), key(v))
and pred(v) = u
Add u in S
For each v in Adj[u]…. can be done in E
complexity
Rest of the loop can be done in V2 complexity
So, overall O(V2)
Using heaps we can solve in O((V + E)logV)
Example
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