Analysis of Algorithms CS 477/677

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Analysis of Algorithms CS 477/677. Instructor: Monica Nicolescu Lecture 19. Graphs. Applications that involve not only a set of items, but also the connections between them. Maps. Schedules. Computer networks. Hypertext. Circuits. 2. 2. 1. 1. 2. 1. 3. 4. 3. 4. 3. 4.

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### Analysis of AlgorithmsCS 477/677

Instructor: Monica Nicolescu

Lecture 19

Graphs
• Applications that involve not only a set of items, but also the connections between them

Maps

Schedules

Computer networks

Hypertext

Circuits

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Graphs - Background

Graphs = a set of nodes (vertices) with edges (links) between them.

Notations:

• G = (V, E) - graph
• V = set of vertices V = n
• E = set of edges E = m

Directed

graph

Undirected

graph

Acyclic

graph

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Other Types of Graphs
• A graph is connected if there is a path between every two vertices
• A bipartite graph is an undirected graph G = (V, E) in which V = V1 + V2 and there are edges only between vertices in V1 and V2

Connected

Not connected

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Graph Representation
• Adjacency list representation of G = (V, E)
• An array of V lists, one for each vertex in V
• Each list Adj[u] contains all the vertices v such that there is an edge between u and v
• Can be used for both directed and undirected graphs

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Undirected graph

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• Sum of the lengths of all the adjacency lists
• Directed graph:
• Edge (u, v) appears only once in u’s list
• Undirected graph:
• u and v appear in each other’s adjacency lists: edge (u, v) appears twice

Directed graph

E 

2 E 

Undirected graph

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• Memory required
• (|V| + |E|)
• Preferred when
• the graph is sparse: E  << V 2
• no quick way to determine whether there is an edge between node u and v
• Time to list all vertices adjacent to u:
• (degree(u))
• Time to determine if (u, v)  E:
• O(degree(u))

Undirected graph

Directed graph

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Graph Representation
• Adjacency matrix representation of G = (V, E)
• Assume vertices are numbered 1, 2, … V 
• The representation consists of a matrix A V x V :
• aij = 1 if (i, j)  E

0 otherwise

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For undirected graphs matrix A is symmetric:

aij = aji

A = AT

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Undirected graph

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CS 477/677 - Lecture 19

• Memory required
• (|V|2), independent on the number of edges in G
• Preferred when
• The graph is dense: E is close to V 2
• We need to quickly determine if there is an edge between two vertices
• Time to list all vertices adjacent to u:
• (|V|)
• Time to determine if (u, v)  E:
• (1)

CS 477/677 - Lecture 19

Weighted Graphs
• Weighted graphs = graphs for which each edge has an associated weight w(u, v)

w:E R, weight function

• Storing the weights of a graph
• Store w(u,v) along with vertex v in u’s adjacency list
• Store w(u, v) at location (u, v) in the matrix

CS 477/677 - Lecture 19

Searching in a Graph
• Graph searching = systematically follow the edges of the graph so as to visit the vertices of the graph
• Two basic graph searching algorithms:
• Depth-first search
• The difference between them is in the order in which they explore the unvisited edges of the graph
• Graph algorithms are typically elaborations of the basic graph-searching algorithms

CS 477/677 - Lecture 19

• Input:
• A graph G = (V, E) (directed or undirected)
• A source vertex s  V
• Goal:
• Explore the edges of G to “discover” every vertex reachable from s, taking the ones closest to s first
• Output:
• d[v] = distance (smallest # of edges) from s to v, for all v  V
• A “breadth-first tree” rooted at s that contains all reachable vertices

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• Keeping track of progress:
• Color each vertex in either white, gray or black
• Initially, all vertices are white
• When being discovered a vertex becomes gray
• After discovering all its adjacent vertices the node becomes black
• Use FIFO queue Q to maintain the set of gray vertices

source

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• BFS constructs a breadth-first tree
• Initially contains the root (source vertex s)
• When vertex v is discovered while scanning the adjacency list of a vertex u vertex v and edge (u, v) are added to the tree
• u is the predecessor (parent) of v in the breadth-first tree
• A vertex is discovered only once  it has only one parent

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• G = (V, E) represented using adjacency lists
• color[u] – the color of the vertex for all u  V
• [u] – predecessor of u
• If u = s (root) or node u has not yet been discovered  [u] = NIL
• d[u] – the distance from the source s to vertex u
• Use a FIFO queue Q to maintain the set of gray vertices

source

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=1

d=2

=2

d=1

=1

d=2

=5

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BFS(V, E, s)
• for each u  V - {s}
• docolor[u] WHITE
• d[u] ← 
• [u] = NIL
• color[s]  GRAY
• d[s] ← 0
• [s] = NIL
• Q 
• Q ← ENQUEUE(Q, s)

Q: s

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BFS(V, E, s)
• while Q 
• do u ← DEQUEUE(Q)
• for each v  Adj[u]
• do if color[v] = WHITE
• thencolor[v] = GRAY
• d[v] ← d[u] + 1
• [v] = u
• ENQUEUE(Q, v)
• color[u] BLACK

Q: s

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Q: w, r

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Example

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Q: 

CS 477/677 - Lecture 19

Analysis of BFS
• for each u  V - {s}
• do color[u]  WHITE
• d[u] ← 
• [u] = NIL
• color[s]  GRAY
• d[s] ← 0
• [s] = NIL
• Q  
• Q ← ENQUEUE(Q, s)

O(|V|)

(1)

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Scan Adj[u] for all vertices u in the graph

• Each vertex u is processed only once, when the vertex is dequeued
• Sum of lengths of all adjacency lists = (|E|)
• Scanning operations: O(|E|)
Analysis of BFS
• while Q 
• do u ← DEQUEUE(Q)
• for each v  Adj[u]
• do if color[v] = WHITE
• thencolor[v] = GRAY
• d[v] ← d[u] + 1
• [v] = u
• ENQUEUE(Q, v)
• color[u] BLACK
• Total running time for BFS = O(|V| + |E|)

(1)

(1)

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Shortest Paths Property
• BFS finds the shortest-path distance from the source vertex s  V to each node in the graph
• Shortest-path distance = (s, u)
• Minimum number of edges in any path from s to u

source

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Depth-First Search
• Input:
• G = (V, E) (No source vertex given!)
• Goal:
• Explore the edges of G to “discover” every vertex in V starting at the most current visited node
• Search may be repeated from multiple sources
• Output:
• 2 timestamps on each vertex:
• d[v] = discovery time
• f[v] = finishing time (done with examining v’s adjacency list)
• Depth-first forest

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Depth-First Search
• Search “deeper” in the graph whenever possible
• Edges are explored out of the most recently discovered vertex v that still has unexplored edges
• After all edges of v have been explored, the search “backtracks” from the parent of v
• The process continues until all vertices reachable from the original source have been discovered
• If undiscovered vertices remain, choose one of them as a new source and repeat the search from that vertex
• DFS creates a “depth-first forest”

CS 477/677 - Lecture 19

1 ≤ d[u] < f [u] ≤ 2 |V|

GRAY

BLACK

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2|V|

• Global variable: time-step
• Incremented when nodes are discovered/finished
• color[u] – similar to BFS
• White before discovery, gray while processing and black when finished processing
• [u] – predecessor of u
• d[u], f[u] – discovery and finish times

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DFS(V, E)
• for each u  V
• do color[u] ← WHITE
• [u] ← NIL
• time ← 0
• for each u  V
• do if color[u] = WHITE
• then DFS-VISIT(u)
• Every time DFS-VISIT(u) is called, u becomes the root of a new tree in the depth-first forest

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DFS-VISIT(u)
• color[u] ← GRAY
• time ← time+1
• d[u] ← time
• for each v  Adj[u]
• do if color[v] = WHITE
• then [v] ← u
• DFS-VISIT(v)
• color[u] ← BLACK
• time ← time + 1
• f[u] ← time

time = 1

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Example

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Example (cont.)
• The results of DFS may depend on:
• The order in which nodes are explored in procedure DFS
• The order in which the neighbors of a vertex are visited in DFS-VISIT

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Edge Classification
• Tree edge (reaches a WHITE vertex):
• (u, v) is a tree edge if v was first discovered by exploring edge (u, v)
• Back edge (reaches a GRAY vertex):
• (u, v), connecting a vertex u to an ancestor v in a depth first tree
• Self loops (in directed graphs) are also back edges

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Edge Classification
• Forward edge (reaches a BLACK vertex & d[u] < d[v]):
• Non-tree edge (u, v) that connects a vertex u to a descendant v in a depth first tree
• Cross edge (reaches a BLACK vertex & d[u] > d[v]):
• Can go between vertices in same depth-first tree (as long as there is no ancestor / descendant relation) or between different depth-first trees

CS 477/677 - Lecture 19

(|V|)

(|V|) – without counting the time for

DFS-VISIT

Analysis of DFS(V, E)
• for each u  V
• do color[u] ← WHITE
• [u] ← NIL
• time ← 0
• for each u  V
• do if color[u] = WHITE
• then DFS-VISIT(u)

CS 477/677 - Lecture 19

Each loop takes

(|E|)

Analysis of DFS-VISIT(u)
• color[u] ← GRAY
• time ← time+1
• d[u] ← time
• for each v  Adj[u]
• do if color[v] = WHITE
• then [v] ← u
• DFS-VISIT(v)
• color[u] ← BLACK
• time ← time + 1
• f[u] ← time

DFS-VISIT is called exactly once for each vertex

= (|V| + |E|)

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Properties of DFS
• u = [v]  DFS-VISIT(v) was called during a search of u’s adjacency list
• Vertex v is a descendant of vertex u in the depth first forest  v is discovered during the time in which u is gray

CS 477/677 - Lecture 19

Parenthesis Theorem

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In any DFS of a graph G, for all u, v, exactly one of the following holds:

• [d[u], f[u]] and [d[v], f[v]] are disjoint, and neither of u and v is a descendant of the other
• [d[v], f[v]] is entirely within [d[u], f[u]] and v is a descendant of u
• [d[u], f[u]] is entirely within [d[v], f[v]] and u is a descendant of v

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properly nested

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Other Properties of DFS

Corollary

Vertex v is a proper descendant of u

 d[u] < d[v] < f[v] < f[u]

Theorem (White-path Theorem)

In a depth-first forest of a graph G, vertex v is a descendant of u if and only if at time d[u], there is a path u  v consisting of only white vertices.

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Topological Sort

Topological sort of a directed acyclic graph G = (V, E): a linear order of vertices such that if there exists an edge (u, v), then u appears before v in the ordering.

• Directed acyclic graphs (DAGs)
• Used to represent precedence of events or processes that have a partial order

a before bb before c

b before c a before c

a and b?

a before c

Topological sort helps us establish a total order

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Topological Sort

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Topological sort:

an ordering of vertices along a

horizontal line so that all directed

edges go from left to right.

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CS 477/677 - Lecture 19

Topological Sort
• TOPOLOGICAL-SORT(V, E)
• Call DFS(V, E) to compute finishing times f[v] for each vertex v
• When each vertex is finished, insert it onto the front of a linked list
• Return the linked list of vertices

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Running time: (|V| + |E|)

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