Upload
gladys-lamb
View
238
Download
0
Tags:
Embed Size (px)
Citation preview
1
Advanced DFS, BFS, Graph Modeling
25/2/2006
2
Introduction
• Depth-first search (DFS)
• Breadth-first search (BFS)
• Graph Modeling– Model a graph from a problem, ie.
transform a problem into a graph problem
3
What is a graph?• A set of vertices and edges
vertex
edge
4
Trees and related terms
root
siblings
descendents children
ancestors
parent
5
What is a tree?• A tree is an undirected simple graph G that satisfies
any of the following equivalent conditions:
– G is connected and has no simple cycles. – G has no simple cycles and, if any edge is added to G, then
a simple cycle is formed. – G is connected and, if any edge is removed from G, then it is
not connected anymore. – Any two vertices in G can be connected by a unique simple
path. – G is connected and has n − 1 edges. – G has no simple cycles and has n − 1 edges.
6
Graph Traversal• Given: a graph• Goal: visit all (or some) vertices and edges of
the graph using some strategy (the order of visit is systematic)
• DFS, BFS are examples of graph traversal algorithms
• Some shortest path algorithms and spanning tree algorithms have specific visit order
7
Idea of DFS and BFS• This is a brief idea of DFS and BFS
• DFS: continue visiting next vertex whenever there is a road, go back if no road (ie. visit to the depth of current path)– Example: a human want to visit a place, but do not know the
path
• BFS: go through all the adjacent vertices before going further (ie. spread among next vertices)– Example: set a house on fire, the fire will spread through the
house
8
DFS (pseudo code)
DFS (vertex u) {mark u as visitedfor each vertex v directly reachable from u
if v is unvisitedDFS (v)
}
• Initially all vertices are marked as unvisited
9
F
A
BC
D
E
DFS (Demonstration)
unvisited
visited
10
“Advanced” DFS
• Apart from just visiting the vertices, DFS can also provide us with valuable information
• DFS can be enhanced by introducing:– birth time and death time of a vertex
• birth time: when the vertex is first visited• death time: when we retreat from the vertex
– DFS tree– parent of a vertex
11
DFS spanning tree / forest• A rooted tree• The root is the start vertex• If v is first visited from u, then u is the parent of v in
the DFS tree• Edges are those in forward direction of DFS, ie. when
visiting vertices that are not visited before
• If some vertices are not reachable from the start vertex, those vertices will form other spanning trees (1 or more)
• The collection of the trees are called forest
12
DFS (pseudo code)
DFS (vertex u) {mark u as visited
time time+1; birth[u]=time;for each vertex v directly reachable from u
if v is unvisitedparent[v]=uDFS (v)
time time+1; death[u]=time;}
13
A
F
B
C
D
E
GH
DFS forest (Demonstration)
unvisited
visited
visited (dead)
A B C D E F G H
birth
death
parent
A
B
C
F
E
D
G
1 2 3 13 10 4 14
12 9 8 16 11 5 15
H
6
7
- A B - A C D C
14
Classification of edges
• Tree edge• Forward edge• Back edge• Cross edge
• Question: which type of edges is always absent in an undirected graph?
A
B
C
F
E
D
G
H
15
Determination of edge types• How to determine the type of an arbitrary
edge (u, v) after DFS?• Tree edge
– parent [v] = u
• Forward edge– not a tree edge; and– birth [v] > birth [u]; and– death [v] < death [u]
• How about back edge and cross edge?
16
Determination of edge types
Tree edge
Forward Edge Back Edge Cross Edge
parent [v] = u
not a tree edge
birth[v] > birth[u]
death[v] < death[u]
birth[v] < birth[u]
death[v] > death[u]
birth[v] < birth[u]
death[v] < death[u]
17
Applications of DFS Forests
• Topological sorting (Tsort)
• Strongly-connected components (SCC)
• Some more “advanced” algorithms
18
Example: Tsort• Topological order: A numbering of the
vertices of a directed acyclic graph such that every edge from a vertex numbered i to a vertex numbered j satisfies i<j
• Tsort: Number the vertices in topological order 1
2
3
4 5
6
7
19
Tsort Algorithm• If the graph has more then one vertex that
has indegree 0, add a vertice to connect to all indegree-0 vertices
• Let the indegree 0 vertice be s• Use s as start vertice, and compute the DFS
forest• The death time of the vertices represent the
reverse of topological order
20
Tsort (Demonstration)
S
D
B
E F
C
G
A
S A B C D E F G
birth
death
G C F B A E D
1 2 3 4 5
67
8
91011
12 13
141516
D E A B F C G
21
Example: SCC• A graph is strongly-connected if
– for any pair of vertices u and v, one can go from u to v and from v to u.
• Informally speaking, an SCC of a graph is a subset of vertices that– forms a strongly-connected subgraph– does not form a strongly-connected
subgraph with the addition of any new vertex
22
SCC (Illustration)
23
SCC (Algorithm)• Compute the DFS forest of the graph G to
get the death time of the vertices• Reverse all edges in G to form G’• Compute a DFS forest of G’, but always
choose the vertex with the latest death time when choosing the root for a new tree
• The SCCs of G are the DFS trees in the DFS forest of G’
24
A
F
B
C
D
GH
SCC (Demonstration)
A
F
B
C
D
E
GH
A B C D E F G H
birth
death
parent
1 2 3 13 10 4 14
12 9 8 16 11 5 15
6
7
- A B - A C D C
D
G
A E B
F
C
H
25
SCC (Demonstration)
D
G
A E B
F
C
H
A
F
B
C
D
GH
E
26
DFS Summary
• DFS spanning tree / forest
• We can use birth time and death time in DFS spanning tree to do varies things, such as Tsort, SCC
• Notice that in the previous slides, we related birth time and death time. But in the discussed applications, birth time and death time can be independent, ie. birth time and death time can use different time counter
27
Breadth-first search (BFS)
• In order to “spread”, we need to makes use of a data structure, queue ,to remember just visited vertices
• Revised:• DFS: continue visiting next
vertex whenever there is a road, go back if no road (ie. visit to the depth of current path)
• BFS: go through all the adjacent vertices before going further (ie. spread among next vertices)
28
BFS (Pseudo code)while queue not empty
dequeue the first vertex u from queue
for each vertex v directly reachable from u
if v is unvisited
enqueue v to queue
mark v as visited
• Initially all vertices except the start vertex are marked as unvisited and the queue contains the start vertex only
29
A
B
C
D
E
F
G
H
I
J
BFS (Demonstration)
unvisited
visited
visited (dequeued)
Queue: A B C F D E H G J I
30
Applications of BFS
• Shortest paths finding
• Flood-fill (can also be handled by DFS)
31
Comparisons of DFS and BFS
DFS BFS
Depth-first Breadth-first
Stack Queue
Does not guarantee shortest paths
Guarantees shortest paths
32
What is graph modeling?
• Conversion of a problem into a graph problem
• Sometimes a problem can be easily solved once its underlying graph model is recognized
• Graph modeling appears almost every year in NOI or IOI
33
Basics of graph modeling
• A few steps:– identify the vertices and the edges– identify the objective of the problem– state the objective in graph terms– implementation:
• construct the graph from the input instance• run the suitable graph algorithms on the graph• convert the output to the required format
34
Simple examples (1)• Given a grid maze with obstacles, find a
shortest path between two given points
start
goal
35
Simple examples (2)
• A student has the phone numbers of some other students
• Suppose you know all pairs (A, B) such that A has B’s number
• Now you want to know Alan’s number, what is the minimum number of calls you need to make?
36
Simple examples (2)
• Vertex: student
• Edge: whether A has B’s number– Add an edge from A to B if A has B’s
number
• Problem: find a shortest path from your vertex to Alan’s vertex
37
Complex examples (1)• Same settings as simple example 1
• You know a trick – walking through an obstacle! However, it can be used for only once
• What should a vertex represent?– your position only?– your position + whether you have used the
trick
38
Complex examples (1)
• A vertex is in the form (position, used)
• The vertices are divided into two groups– trick used– trick not used
39
Complex examples (1)
start
goal
unused
used
start goal
goal
40
Complex examples (1)
• How about you can walk through obstacles for k times?
41
Complex examples (1)
k
k-1
start goal
k-2
42
Complex examples (1)
k
k-1
start goal
k-4 k-2k-3
43
Complex examples (2)• The famous 8-puzzle
• Given a state, find the moves that bring it to the goal state
1 2 3
4 5 6
7 8
44
Complex examples (2)• What does a vertex represent?
– the position of the empty square?– the number of tiles that are in wrong
positions?– the state (the positions of the eight tiles)
• What are the edges?
• What is the equivalent graph problem?
45
Complex examples (2)1 2 34 5 67 8
1 2 34 5 67 8
1 2 34 57 8 6
1 2 34 67 5 8
1 2 34 5 6
7 8
1 24 5 37 8 6
1 2 34 57 8 6
46
Complex examples (3)• Theseus and Minotaur
– http://www.logicmazes.com/theseus.html– Extract:
• Theseus must escape from a maze. There is also a mechanical Minotaur in the maze. For every turn that Theseus takes, the Minotaur takes two turns. The Minotaur follows this program for each of his two turns:
• First he tests if he can move horizontally and get closer to Theseus. If he can, he will move one square horizontally. If he can’t, he will test if he could move vertically and get closer to Theseus. If he can, he will move one square vertically. If he can’t move either horizontally or vertically, then he just skips that turn.
47
Complex examples (3)• What does a vertex represent?
– Theseus’ position– Minotaur’s position– Both
48
Some more examples• How can the followings be modeled?
– Tilt maze (Single-goal mazes only)• http://www.clickmazes.com/newtilt/ixtilt2d.htm
– Double title maze• http://www.clickmazes.com/newtilt/ixtilt.htm
– No-left-turn maze• http://www.clickmazes.com/noleft/ixnoleft.htm
– Same as complex example 1, but you can use the trick for k times
49
Competition problems• HKOI2000 S – Wormhole Labyrinth• HKOI2001 S – A Node Too Far• HKOI2004 S – Teacher’s Problem *• TFT2001 – OIMan * • TFT2002 – Bomber Man *• NOI2001 – cung1 ming4 dik7 daa2 zi6 jyun4• NOI2001 – Equation *• IOI2000 – Walls • IOI2002 – Troublesome Frog• IOI2003 – Amazing Robots
50
Teacher’s Problem
• Question: A teacher wants to distribute sweets to students in an order such that, if student u tease student v, u should not get the sweet before v
• Vertex: student• Edge: directed, (v,u) is a directed edge if
student v tease u• Algorithm: Tsort
51
OI Man• Question: OIMan (O) has 2 kinds of form:
H- and S-form. He can transform to S-form for m minutes by battery. He can only kill monsters (M) and virus (V) in S-form. Given the number of battery and m, find the minimum time needed to kill the virus.
• Vertex: position, form, time left for S-form, number of batteries left
• Edge: directed– Move to P in H-form (position)– Move to P/M/V in S-form (position, S-form time)– Use battery and move (position, form, S-form
time, number of batteries)
PWWPPPPPPWWPOMMV
52
What you have learnt:
Graph Modeling
53
Variations of BFS and DFS
• Bidirectional Search (BDS)
• Iterative Deepening Search(IDS)
54
Bidirectional search (BDS)
• Searches simultaneously from both the start vertex and goal vertex
• Commonly implemented as bidirectional BFS
start goal
55
BDS Example: Bomber Man (1 Bomb)
• find the shortest path from the upper-left corner to the lower-right corner in a maze using a bomb. The bomb can destroy a wall.
S
E
56
Bomber Man (1 Bomb)
S
E
1 2 3
4
1234
5
4
Shortest Path length = 8
5
6 67 78 8
9
9
10
10
11
11
12
12 12
13
13
What will happen if we stop once we find a path?
57
S 1 2 3 4 5 6 7 8 9
10
21 11
20 12
19 13
18 14
17 17 16 15
11 12 13 14 15 16
10
9 8 7 6 5 4 3 2 1 E
Example
58
Iterative deepening search (IDS)
• Iteratively performs DFS with increasing depth bound
• Shortest paths are guaranteed
59
IDS
60
IDS (pseudo code)DFS (vertex u, depth d) {
mark u as visitedif (d>0)
for each vertex v directly reachable from uif v is unvisited
DFS (v,d-1)}
i=0Do {
DFS(start vertex,i)Increment i
}While (target is not found)
61
IDS• The complexity of IDS is the same as
DFS
62
Summary of DFS, BFS
• We learned some variations of DFS and BFS – Bidirectional search (BDS) – Iterative deepening search (IDS)
63
Equation
• Question: Find the number of solution of xi, given ki,pi. 1<=n<=6, 1<=xi<=150
• Vertex: possible values of – k1x1
p1 , k1x1p1 + k2x2
p2 , k1x1p1 + k2x2
p2 + k3x3p3 , k4x4
p4 , k4x4
p4 + k5x5p5 , k4x4
p4 + k5x5p5 + k6x6
p6