Breadth-First Search
3/25/14 16:17
Presentation for use with the textbook Data Structures and
Algorithms in Java, 6th edition, by M. T. Goodrich, R. Tamassia,
and M. H. Goldwasser, Wiley, 2014
Breadth-First Search
L0
L1
A
B
C
L2
© 2014 Goodrich, Tamassia, Goldwasser
E
D
F
Breadth-First Search
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Breadth-First Search
q 
q 
Breadth-first search
(BFS) is a general
technique for traversing
a graph
A BFS traversal of a
graph G
n 
n 
n 
n 
Visits all the vertices and
edges of G
Determines whether G is
connected
Computes the connected
components of G
Computes a spanning
forest of G
© 2014 Goodrich, Tamassia, Goldwasser
q 
q 
BFS on a graph with n
vertices and m edges
takes O(n + m ) time
BFS can be further
extended to solve other
graph problems
n 
n 
Breadth-First Search
Find and report a path
with the minimum
number of edges
between two given
vertices
Find a simple cycle, if
there is one
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BFS Algorithm
q 
The algorithm uses a
mechanism for setting and
getting “labels” of vertices
and edges
Algorithm BFS(G)
Input graph G
Output labeling of the edges
and partition of the
vertices of G
for all u ∈ G.vertices()
setLabel(u, UNEXPLORED)
for all e ∈ G.edges()
setLabel(e, UNEXPLORED)
for all v ∈ G.vertices()
if getLabel(v) = UNEXPLORED
BFS(G, v)
© 2014 Goodrich, Tamassia, Goldwasser
Algorithm BFS(G, s)
L0 ← new empty sequence
L0.addLast(s)
setLabel(s, VISITED)
i←0
while ¬Li.isEmpty()
Li +1 ← new empty sequence
for all v ∈ Li.elements()
for all e ∈ G.incidentEdges(v)
if getLabel(e) = UNEXPLORED
w ← opposite(v,e)
if getLabel(w) = UNEXPLORED
setLabel(e, DISCOVERY)
setLabel(w, VISITED)
Li +1.addLast(w)
else
setLabel(e, CROSS)
i ← i +1
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Java Implementation
© 2014 Goodrich, Tamassia, Goldwasser
Breadth-First Search
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Example
unexplored vertex
visited vertex
unexplored edge
discovery edge
cross edge
A
A
L0
L1
L0
L1
B
L0
C
E
D
L1
D
F
A
B
F
© 2014 Goodrich, Tamassia, Goldwasser
C
E
A
B
A
C
E
D
F
Breadth-First Search
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Example (cont.)
L0
L1
L0
A
B
C
E
L0
L1
L1
B
L2
F
L0
A
B
L2
D
C
E
© 2014 Goodrich, Tamassia, Goldwasser
D
L1
Breadth-First Search
C
E
D
F
A
B
L2
F
A
C
E
D
F
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Example (cont.)
L0
L1
B
L2
C
E
L0
L1
D
L1
B
L2
F
A
C
E
D
F
A
B
L2
L0
A
C
E
D
F
© 2014 Goodrich, Tamassia, Goldwasser
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Properties
Notation
A
Gs: connected component of s
Property 1
BFS(G, s) visits all the vertices and
edges of Gs
B
E
Property 2
The discovery edges labeled by
BFS(G, s) form a spanning tree Ts
of Gs
Property 3
For each vertex v in Li
n 
n 
L0
L1
The path of Ts from s to v has i
edges
Every path from s to v in Gs has at
least i edges
© 2014 Goodrich, Tamassia, Goldwasser
Breadth-First Search
C
F
A
B
L2
D
C
E
D
F
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Analysis
q 
q 
Setting/getting a vertex/edge label takes O(1) time
Each vertex is labeled twice
n 
n 
q 
Each edge is labeled twice
n 
n 
q 
q 
q 
once as UNEXPLORED
once as VISITED
once as UNEXPLORED
once as DISCOVERY or CROSS
Each vertex is inserted once into a sequence Li
Method incidentEdges is called once for each vertex
BFS runs in O(n + m) time provided the graph is
represented by the adjacency list structure
n 
Recall that
Σv deg(v) = 2m
© 2014 Goodrich, Tamassia, Goldwasser
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Applications
q 
Using the template method pattern, we can
specialize the BFS traversal of a graph G to
solve the following problems in O(n + m) time
n 
n 
n 
n 
Compute the connected components of G
Compute a spanning forest of G
Find a simple cycle in G, or report that G is a
forest
Given two vertices of G, find a path in G between
them with the minimum number of edges, or
report that no such path exists
© 2014 Goodrich, Tamassia, Goldwasser
Breadth-First Search
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DFS vs. BFS
Applications
DFS
BFS
√
√
Spanning forest, connected
components, paths, cycles
Shortest paths
√
Biconnected components
√
L0
A
B
C
E
D
L1
A
B
L2
F
DFS
C
E
D
F
BFS
© 2014 Goodrich, Tamassia, Goldwasser
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DFS vs. BFS (cont.)
Back edge (v,w)
n 
Cross edge (v,w)
w is an ancestor of v in
the tree of discovery
edges
n 
L0
A
B
w is in the same level as
v or in the next level
C
E
D
L1
DFS
© 2014 Goodrich, Tamassia, Goldwasser
B
L2
F
A
C
E
D
F
BFS
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