The document discusses minimum spanning trees and provides examples of Prim's and Kruskal's algorithms. It includes: - A definition of minimum spanning tree as a subgraph that spans all nodes with minimum total edge weight. - Characteristics of Prim's and Kruskal's algorithms such as working with undirected, weighted/unweighted graphs and producing optimal solutions greedily. - A walk-through example of Prim's algorithm on a graph and calculating the minimum spanning tree cost.

1. Data Structures and
Algorithms
Week 8: Minimum Spanning Trees
Ferdin Joe John Joseph, PhD
Faculty of Information Technology
Thai-Nichi Institute of Technology, Bangkok

2. Week 8
• Minimum Spanning Trees
• Implementation in Java
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3. Definition
• A Minimum Spanning Tree (MST) is a subgraph of
an undirected graph such that the subgraph spans
(includes) all nodes, is connected, is acyclic, and
has minimum total edge weight
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4. Algorithm Characteristics
• Both Prim’s and Kruskal’s Algorithms work with
undirected graphs
• Both work with weighted and unweighted graphs but
are more interesting when edges are weighted
• Both are greedy algorithms that produce optimal
solutions
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5. Prim’s Algorithm
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6. Walk-Through
Initialize array
K dv pv
A F ∞ −
B F ∞ −
C F ∞ −
D F ∞ −
E F ∞ −
F F ∞ −
G F ∞ −
H F ∞ −
4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
2
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7. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Start with any node, say D
K dv pv
A
B
C
D T 0 −
E
F
G
H
2
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8. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Update distances of
adjacent, unselected
nodes
K dv pv
A
B
C 3 D
D T 0 −
E 25 D
F 18 D
G 2 D
H
2
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9. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Select node with minimum
distance
K dv pv
A
B
C 3 D
D T 0 −
E 25 D
F 18 D
G T 2 D
H
2
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10. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Update distances of
adjacent, unselected
nodes
K dv pv
A
B
C 3 D
D T 0 −
E 7 G
F 18 D
G T 2 D
H 3 G
2
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11. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Select node with minimum
distance
K dv pv
A
B
C T 3 D
D T 0 −
E 7 G
F 18 D
G T 2 D
H 3 G
2
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12. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Update distances of
adjacent, unselected
nodes
K dv pv
A
B 4 C
C T 3 D
D T 0 −
E 7 G
F 3 C
G T 2 D
H 3 G
2
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13. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Select node with minimum
distance
K dv pv
A
B 4 C
C T 3 D
D T 0 −
E 7 G
F T 3 C
G T 2 D
H 3 G
2
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14. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Update distances of
adjacent, unselected
nodes
K dv pv
A 10 F
B 4 C
C T 3 D
D T 0 −
E 2 F
F T 3 C
G T 2 D
H 3 G
2
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15. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Select node with minimum
distance
K dv pv
A 10 F
B 4 C
C T 3 D
D T 0 −
E T 2 F
F T 3 C
G T 2 D
H 3 G
2
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16. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Update distances of
adjacent, unselected
nodes
K dv pv
A 10 F
B 4 C
C T 3 D
D T 0 −
E T 2 F
F T 3 C
G T 2 D
H 3 G
2
Table entries unchanged
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17. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Select node with minimum
distance
K dv pv
A 10 F
B 4 C
C T 3 D
D T 0 −
E T 2 F
F T 3 C
G T 2 D
H T 3 G
2
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18. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Update distances of
adjacent, unselected
nodes
K dv pv
A 4 H
B 4 C
C T 3 D
D T 0 −
E T 2 F
F T 3 C
G T 2 D
H T 3 G
2
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18

19. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Select node with minimum
distance
K dv pv
A T 4 H
B 4 C
C T 3 D
D T 0 −
E T 2 F
F T 3 C
G T 2 D
H T 3 G
2
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20. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Update distances of
adjacent, unselected
nodes
K dv pv
A T 4 H
B 4 C
C T 3 D
D T 0 −
E T 2 F
F T 3 C
G T 2 D
H T 3 G
2
Table entries unchanged
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Algorithms, Data Science and Analytics,
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21. 4
25
A
H
B
F
E
D
C
G 7
2
10
18
3
4
3
7
8
9
3
10
Select node with minimum
distance
K dv pv
A T 4 H
B T 4 C
C T 3 D
D T 0 −
E T 2 F
F T 3 C
G T 2 D
H T 3 G
2
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22. 4
A
H
B
F
E
D
C
G
2
3
4
3
3
Cost of Minimum
Spanning Tree = Σ dv = 21
K dv pv
A T 4 H
B T 4 C
C T 3 D
D T 0 −
E T 2 F
F T 3 C
G T 2 D
H T 3 G
2
Done
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23. Java Implementation
https://www.geeksforgeeks.org/prims-minimum-
spanning-tree-mst-greedy-algo-5/
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24. Kruskal’s Algorithm
Work with edges, rather than nodes
Two steps:
– Sort edges by increasing edge weight
– Select the first |V| – 1 edges that do not
generate a cycle
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25. Walk-Through
Consider an undirected, weight graph
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10
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26. Sort the edges by increasing edge weight
edge dv
(D,E) 1
(D,G) 2
(E,G) 3
(C,D) 3
(G,H) 3
(C,F) 3
(B,C) 4
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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27. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2
(E,G) 3
(C,D) 3
(G,H) 3
(C,F) 3
(B,C) 4
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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28. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3
(C,D) 3
(G,H) 3
(C,F) 3
(B,C) 4
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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29. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3
(G,H) 3
(C,F) 3
(B,C) 4
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
Accepting edge (E,G) would create a cycle
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30. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3
(C,F) 3
(B,C) 4
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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31. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3 √
(C,F) 3
(B,C) 4
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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32. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3 √
(C,F) 3 √
(B,C) 4
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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33. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3 √
(C,F) 3 √
(B,C) 4 √
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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34. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3 √
(C,F) 3 √
(B,C) 4 √
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4 χ
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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35. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3 √
(C,F) 3 √
(B,C) 4 √
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4 χ
(B,F) 4 χ
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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36. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3 √
(C,F) 3 √
(B,C) 4 √
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4 χ
(B,F) 4 χ
(B,H) 4 χ
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10
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37. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3 √
(C,F) 3 √
(B,C) 4 √
5
1
A
H
B
F
E
D
C
G 3
2
4
6
3
4
3
4
8
4
3
10 edge dv
(B,E) 4 χ
(B,F) 4 χ
(B,H) 4 χ
(A,H) 5 √
(D,F) 6
(A,B) 8
(A,F) 10
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38. Select first |V|–1 edges which do not
generate a cycle
edge dv
(D,E) 1 √
(D,G) 2 √
(E,G) 3 χ
(C,D) 3 √
(G,H) 3 √
(C,F) 3 √
(B,C) 4 √
5
1
A
H
B
F
E
D
C
G
2
3
3
3
edge dv
(B,E) 4 χ
(B,F) 4 χ
(B,H) 4 χ
(A,H) 5 √
(D,F) 6
(A,B) 8
(A,F) 10
Done
Total Cost = Σ dv = 21
4
}not
considere
d
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39. Java Implementation
https://www.geeksforgeeks.org/kruskals-minimum-
spanning-tree-algorithm-greedy-algo-2/
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40. Next Week
Search Algorithms
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