The document discusses two algorithms for finding minimum spanning trees: Prim's algorithm and Kruskal's algorithm. Prim's algorithm is similar to Dijkstra's algorithm for finding shortest paths. It works by gradually adding the closest vertex to the growing spanning tree. Kruskal's algorithm focuses on edges rather than vertices. It sorts the edges by weight and builds the spanning tree by adding the lowest weight edges that do not create cycles. Both algorithms find optimal minimum spanning trees for weighted and unweighted graphs.

1. MINIMUM SPANNING TREES
Text
• Read Weiss, §9.5
Prim’s Algorithm
• Weiss §9.5.1
• Similar to Dijkstra’s Algorithm
Kruskal’s Algorithm
• Weiss §9.5.2
• Focuses on edges, rather than nodes

2. 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

3. 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

4. PRIM’S ALGORITHM
• Similar to Dijkstra’s Algorithm except that dv records edge weights, not path
lengths

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

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

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

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

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 χ
(B,F) 4
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10

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 χ
(B,F) 4 χ
(B,H) 4
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10

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 χ
(B,H) 4 χ
(A,H) 5
(D,F) 6
(A,B) 8
(A,F) 10

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 χ
(A,H) 5 √
(D,F) 6
(A,B) 8
(A,F) 10

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
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
considered

37. DETECTING CYCLES
• Use Disjoint Sets (Chapter 8)