Spanning trees

GRAPHS
 Graphs are one of the most Interesting data structures in computer science.
 Graphs and trees are somewhat similar by their structure and in fact tree is derived from the
graph data structure.
 Commonly used graph traversal algorithms are:
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BFS DFS
In this we visit the nodes level by level,
so it will start with level 0, which is the
root node then next, then the last level.
In this we visit the root node first then its
children until it reaches the end node.
Queue is used to implement BFS.
E.G.
Its BFS will be A,B,C,D,E,F.
Stack is used to implement DFS.
E.G.
Its DFS will be A,B,E,F,C,D

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Spanning trees
 Suppose you have a connected undirected graph
 Connected: every node is reachable from every other node.
 Undirected: edges do not have an associated direction
 ...thus in the Mathematical field of graph theory, a spanning tree of the graph is
a connected sub-graph in which there are no cycles.
 A Spanning tree for a graph has no cycles but still connects to every house.
 If G is a connected graph with n vertices and m edges, spanning tree of G must
have n-1 edges, and no. of edges deleted from G to get a spanning tree must be
m-(n-1)=m-n+1.
 A Graph may have many spanning trees; for instance the complete graph on
four vertices.
A connected,
undirected graph
Four of the spanning trees of the graph

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Finding a spanning tree
 To find a spanning tree of a graph,
Pick an initial node and call it part of the spanning tree
do a search from the initial node:
each time you find a node that is not in the spanning tree, add to the
spanning tree both the new node and the edge you followed to get to it.
An undirected graph One possible
result of a BFS
starting from top
One possible
result of a DFS
starting from top

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Minimizing costs
 Suppose you want to supply a set of houses (say, in a
new subdivision) with:
 electric power
 water
 sewage lines
 telephone lines
 To keep costs down, you could connect these houses
with a spanning tree (of, for example, power lines)
 However, the houses are not all equal distances apart
 To reduce costs even further, you could connect the
houses with a minimum-cost spanning tree

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Minimum-cost spanning trees
 Suppose you have a connected undirected graph with a weight (or cost)
associated with each edge
 The cost or weight of a spanning tree would be the sum of the costs of its
edges.
 A minimum-cost spanning tree is a spanning tree of a connected undirected
graph that has the lowest cost.
 If there are n vertices in the graph , then each spanning tree has n vertices & n-
1 edges.
A B
E D
F C
16
19
21 11
33 14
18
10
6
5
A connected, undirected graph
A B
E D
F C
16
11
18
6
5
A minimum-cost spanning tree

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Finding minimum spanning trees
 There are two basic algorithms for finding minimum-cost
spanning trees, and both are greedy algorithms i.e., these
always processes the edge with the least weight or cost.
 Kruskal’s algorithm: Start with no nodes or edges in the
spanning tree, and repeatedly add the cheapest edge that does
not create a cycle
 Here, we consider the spanning tree to consist of edges only
 Prim’s algorithm: Start with any one node in the spanning
tree, and repeatedly add the cheapest edge, and the node it leads
to, for which the node is not already in the spanning tree.
 Here, we consider the spanning tree to consist of both nodes and edges

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Kruskal’s algorithm
Input :A Connected weighted graph G
Output :A minimal spanning tree T.
T = empty spanning tree;
E = set of edges;
N = number of nodes in graph;
while T has fewer than N - 1 edges {
remove an edge (v, w) of lowest cost from E
if adding (v, w) to T would create a cycle
then discard (v, w)
else add (v, w) to T
}
 Finding an edge of lowest cost can be done just by sorting
the edges

E.G of Krusal’s Algorithm
 Find a minimum spanning tree of the below connected undirected
graph by Krusals algorithm.
 List the edges in non-decreasing order
shown below:
 Therefore the minimum spanning tree that will be
produced will be:
9

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Prim’s algorithm
Input :A connected weighted Graph G.
Output :A minimum spanning tree T.
T = a spanning tree containing a single node s;
E = set of edges adjacent to s;
while T does not contain all the nodes {
remove an edge (v, w) of lowest cost from E
if w is already in T then discard edge (v, w)
else {
add edge (v, w) and node w to T
add to E the edges adjacent to w
}
}

E.G OF PRIM’S ALGORITHM
 Find a minimum spanning tree by using prim’s algorithm of the below given graph:
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DIFFERENCE
PRIM’S ALGORITHM
 In this, at every step
edges of minimum
weight that are
incident to a vertex
already in the tree and
not forming a cycle is
choosen .
KRUSAL’S ALGOTITHM
 In this, at every step
edges of minimum
weight that are not
necessarily incident to
a vertex already in the
tree and not forming a
cycle is choosen .
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Spanning trees

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Spanning trees