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**Graphs - Definition G(V,E) - graph with vertex set V and edge set E**

E {(a,b)| aV and bV} - for directed graphs E {{a,b}| aV and bV} - for undirected graphs w: E R - weight function |V| - number of vertices |E| - number of edges Often we will assume that V = {1, ,n}

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Graphs - Examples 6 1 6 1 2 2 3 4 5 3 4 5

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Graphs - Trees 6 1 6 1 2 4 2 4 3 5 3 5

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**Graphs - Directed Acyclic Graphs (DAG)**

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**Graphs - Representations - Adjacency matrix**

1 2 3 4 5 6 6 1 2 3 4 5 6 1 1 2 3 4 5

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**Graphs - Representations - Adjacency lists**

6 1 1 2 3 4 5 6 6 2 6 2 3 3 4 5 1 3 5 2 1

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**Breadth-First Search - Algorithm**

BreadthFirstSearch(graph G, vertex s) for u V[G] {s} do colour[u] white; d[u] ; p[u] 0 colour[s] gray; d[s] 0; p[s] 0 Q {s} while Q 0 do u Head[Q] for v Adj[u] do if colour[v] = white then colour[v] gray; d[v] d[u] + 1; p[v] u EnQueue(Q,v) DeQueue(Q) colour[u] black

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**Breadth-First Search - Example**

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**Breadth-First Search - Example**

d e f g s Q

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**Breadth-First Search - Example**

1 1 d e f g e a Q

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**Breadth-First Search - Example**

1 2 1 2 d e f g a b f Q

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**Breadth-First Search - Example**

1 2 2 1 2 d e f g b f d Q

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**Breadth-First Search - Example**

1 2 3 2 1 2 d e f g f d c Q

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**Breadth-First Search - Example**

1 2 3 2 1 2 3 d e f g d c g Q

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**Breadth-First Search - Example**

1 2 3 2 1 2 3 d e f g c g Q

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**Breadth-First Search - Example**

1 2 3 2 1 2 3 d e f g g Q

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**Breadth-First Search - Example**

1 2 3 2 1 2 3 d e f g Q =

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**Breadth-First Search - Complexity**

BreadthFirstSearch(graph G, vertex s) for u V[G] {s} do colour[u] white; d[u] ; p[u] 0 colour[s] gray; d[s] 0; p[s] 0 Q {s} while Q 0 do u Head[Q] for v Adj[u] do if colour[v] = white then colour[v] gray; d[v] d[u] + 1; p[v] u EnQueue(Q,v) DeQueue(Q) colour[u] black (V) Thus T(V,E)=(V+E) (V) without for cycle (E) for all while cycles together

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**Breadth-First Search - Shortest Distances**

Theorem After BreadthFirstSearch algorithm terminates d[v] is equal with shortest distance from s to v for all vertices v for all vertices v reachable from s the one of the shortest paths from s to v contains edge (p[v], v)

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**Depth-First Search - Algorithm**

DepthFirstSearch(graph G) for u V[G] do colour[u] white p[u] 0 time 0 if colour[v] = white then DFSVisit(v)

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**Depth-First Search - Algorithm**

DFSVisit(vertex u) time time + 1 d[u] time colour[u] gray for v Adj[u] do if colour[v] = white then p[v] u DFSVisit(v) colour[u] black f[u] time

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**Depth-First Search - Example**

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**Depth-First Search - Example**

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**Depth-First Search - Example**

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**Depth-First Search - Example**

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**Depth-First Search - Example**

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**Depth-First Search - Example**

b 1/ 2/ B 4/ 3/ c d e

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**Depth-First Search - Example**

b 1/ 2/ B 4/5 3/ c d e

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**Depth-First Search - Example**

b 1/ 2/ B 4/5 3/6 c d e

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**Depth-First Search - Example**

b 1/ 2/7 B 4/5 3/6 c d e

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**Depth-First Search - Example**

b 1/ 2/7 B F 4/5 3/6 c d e

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**Depth-First Search - Example**

b 1/8 2/7 B F 4/5 3/6 c d e

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**Depth-First Search - Example**

b 1/8 2/7 9/ B F 4/5 3/6 c d e

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**Depth-First Search - Example**

b 1/8 2/7 9/ B C F 4/5 3/6 c d e

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**Depth-First Search - Example**

b 1/8 2/7 9/ B C F 4/5 3/6 10/ c d e

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**Depth-First Search - Example**

b 1/8 2/7 9/ B C F B 4/5 3/6 10/ c d e

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**Depth-First Search - Example**

b 1/8 2/7 9/ B C F B 4/5 3/6 10/11 c d e

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**Depth-First Search - Example**

b 1/8 2/7 9/12 B C F B 4/5 3/6 10/11 c d e

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**Depth-First Search - Complexity**

DepthFirstSearch(graph G) for u V[G] do colour[u] white p[u] 0 time 0 if colour[v] = white then DFSVisit(v) (V) executed (V) times DFSVisit(vertex u) time time + 1 d[u] time colour[u] gray for v Adj[u] do if colour[v] = white then p[v] u DFSVisit(v) colour[u] black f[u] time (E) for all DFSVisit calls together Thus T(V,E)=(V+E)

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**Depth-First Search - Classification of Edges**

Trees edges - edges in depth-first forest Back edges - edges (u, v) connecting vertex u to an v in a depth-first tree (including self-loops) Forward edges - edges (u, v) connecting vertex u to a descendant v in a depth-first tree Cross edges - all other edges

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**Depth-First Search - Classification of Edges**

Theorem In a depth-first search of an undirected graph G, every edge of G is either a tree edge or a back edge.

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**Depth-First Search - White Path Theorem**

If during depth-first search a “white” vertex u is reachable from a “grey” vertex v via path that contains only “white” vertices, then vertex u will be a descendant on v in depth-first search forest.

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**Depth-First Search - Timestamps**

Parenthesis Theorem After DepthFirstSearch algorithm terminates for any two vertices u and v exactly one from the following three conditions holds the intervals [d[u],f[u]] and [d[v],f[v]] are entirely disjoint the intervals [d[u],f[u]] is contained entirely within the interval [d[v],f[v]] and u is a descendant of v in depth- first tree the intervals [d[v],f[v]] is contained entirely within the interval [d[u],f[u]] and v is a descendant of u in depth-

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**Depth-First Search - Timestamps**

b s c 3/6 2/9 1/10 11/16 B F C B 4/5 7/8 12/13 14/15 C C C d e f g

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**Depth-First Search - Timestamps**

b f g e a d (s (b (a (d d) a) (e e) b) s) (c (f f) (g g) c)

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**Depth-First Search - Timestamps**

B F b f g C a e C B C d

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**DFS - Checking for cycles**

[Adapted from M.Golin]

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**DFS - Checking for cycles**

[Adapted from M.Golin]

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**DFS - Checking for cycles**

[Adapted from M.Golin]

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**DFS - Checking for cycles**

[Adapted from M.Golin]

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**DFS - Topological Sorting**

undershorts socks watch pants shoes belt shirt tie jacket

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**DFS - Topological Sorting**

[Adapted from M.Golin]

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**DFS - Topological Sorting**

[Adapted from M.Golin]

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**DFS - Topological Sorting**

TopologicalSort(graph G) call DFS(G) to compute f[v] for all vertices v as f[v] for vertex v is computed, insert onto the front of a linked list return the linked list of vertices

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**DFS - Topological Sorting - Example 1**

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**DFS - Topological Sorting - Example 1**

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**DFS - Topological Sorting - Example 2**

[Adapted from M.Golin]

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**DFS - Topological Sorting**

Theorem TopologicalSort(G) produces a topological sort of a directed acyclic graph G.

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**DFS - Strongly Connected Components**

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**DFS - Strongly Connected Components**

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**DFS - Strongly Connected Components**

[Adapted from L.Joskowicz]

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**DFS - Strongly Connected Components**

[Adapted from L.Joskowicz]

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**DFS - Strongly Connected Components**

[Adapted from L.Joskowicz]

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**DFS - Strongly Connected Components**

[Adapted from L.Joskowicz]

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**DFS - Strongly Connected Components**

StronglyConnectedComponents(graph G) call DFS(G) to compute f[v] for all vertices v compute GT call DFS(GT) consider vertices in order of decreasing of f[v] output the vertices of each tree in the depth-first forest as a separate strongly connected component

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**DFS - Strongly Connected Components**

13/14 11/16 1/10 8/9 12/15 3/4 2/7 5/6

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**DFS - Strongly Connected Components**

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**DFS - Strongly Connected Components**

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DFS - SCC - Correctness 13/14 11/16 1/10 8/9 12/15 3/4 2/7 5/6

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DFS - SCC - Correctness y' y x C(x) Assume that y preceded by y' is the closest vertex to x outside C(x). Then: - d(y)<f(y)<d(x)<d(y) (otherwise we will have xy (in G). - for all x'C(x): d(x)<d(x')<f(x')<f(x) (the largest value of f(x) will have the vertex first "discovered" in C(x)). - thus we have d(y)<f(y)<d(y')<f(y'), however there is and edge (y,y') in G, implying f(y)<d(y') d(y')<y(y). Contradiction.

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**DFS - SCC - Correctness Lemma**

If two vertices are in the same strongly connected, then no path between them leaves this strongly connected component. Theorem In any depth-first search, all vertices in the same strongly connected component are placed in the same depth-first tree.

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**DFS - SCC - Correctness Theorem**

In a directed graph G = (V,E) the forefather (u) of any vertex uV in any depth-first search of G is an ancestor of u. Corollary In any depth-first search of a directed graph G = (V,E) for all uV vertices u and (u) lie in the same strongly connected component.

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**DFS - SCC - Correctness Theorem**

In a directed graph G = (V,E) two vertices u,vV lie in the same strongly connected component if and only if they have the same forefather in a depth-first search of G. StronglyConnectedComponents(G) correctly computes the strongly connected components of a directed graph G.

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DFS - SCC - Correctness 2 [Adapted from S.Whitesides]

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DFS - SCC - Correctness 2 [Adapted from S.Whitesides]

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**DFS - SCC - Applications**

[Adapted from L.Joskowicz]

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