# Informatique Fondamentale IMA S8

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1 Informatique Fondamentale IMA S8 Cours 4 : graphs, problems and algorithms on graphs, (notions of) NP completeness Laure Gonnord Université Lille 1 - Polytech Lille March/April 2011

2 IV - Graphs, problems and algorithms on graphs, NP-completeness Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

3 Some generalities on graphs 1 Some generalities on graphs Generalities and first definitions Internal representation 2 Paths finding - search, and applications 3 Algorithms specific to oriented graphs 4 More difficult problems on graphs 5 NP - completeness 6 Docs Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

4 Some generalities on graphs Generalities and first definitions Goal Model : relationships between objects ( I know him, This register is in conflict with with one, there is a road between these two towns ) classical problems of paths ( Do I have a friend that knows a friend... who knows the Dalai-Lama?, It is possible to go from this town to this another in less than 600 km?, how to get out of the labyrinth? ) Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

5 Some generalities on graphs Generalities and first definitions Who? Euler (1740) formalised the theory and the Konisberg problem : Problem : compute an eulerian path. Source (fr) : Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

6 Some generalities on graphs Generalities and first definitions Other classical problems The travelling salesman problem : TSP Task scheduling (on parallel machines or not!), the PERT method. Coloring maps Covering (telecom) trees Minimal paths... Graphs are everywhere. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

7 Some generalities on graphs Generalities and first definitions General definition Graph A finite set of vertices (a vertex) : V A finite set of edges. E V V v 1 v3 e 1 e 3 v2 A simplier model than the previous ones! Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

8 Some generalities on graphs Generalities and first definitions Other definitions Non Oriented Graph iff (v1, v2), (v1, v2) E (v2, v1) E. Tree (non oriented acyclic connex) - Forest. Directed Acyclic Graph : DAG. Oriented Graph with no cycle. trees are dags! v1 v3 v2 v4 Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

9 Some generalities on graphs Internal representation Sources Picture and Java codes from Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

10 Some generalities on graphs Internal representation Adjacency matrix If the graph is oriented, then the matrix is NOT symmetric Memory complexity : O(V 2 ) Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

11 Some generalities on graphs Internal representation Adjacency matrix - Java boolean [ ] [ ] m; Graph ( i n t n ) { m = new boolean [ n ] [ n ] ; for ( i n t i = 0; i < n ; ++ i ) for ( i n t j = 0; j < n ; ++ j ) m[ x ] [ y ] = f a l s e ; } s t a t i c void addedge ( Graph g, i n t x, i n t y ) { g.m[ x ] [ y ] = t r u e ; g.m[ y ] [ x ] = t r u e / / only i f non o r i e n t e d Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

12 Some generalities on graphs Internal representation Array of lists Memory complexity : O(V + E) Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

13 Some generalities on graphs Internal representation Array of lists - Java L i s t e [ ] succ ; Graph ( i n t n ) { succ = new L i s t e [ n ] ; } } s t a t i c void addedge ( Graph g, i n t x, i n t y ) { g. succ [ x ] = new L i s t e ( y, g. succ [ x ] ) ; / / i f non o r i e n t e d also modify g. succ [ y ]! } Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

14 Paths finding - search, and applications 1 Some generalities on graphs 2 Paths finding - search, and applications Looking for all nodes Connexity Topological Sort 3 Algorithms specific to oriented graphs 4 More difficult problems on graphs 5 NP - completeness 6 Docs Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

15 Paths finding - search, and applications Looking for all nodes Search Algorithms! Translation problem! (en) Search Algorithms - (fr) Parcours de Graphes Reminder : search/do algorithm on a binary tree : 1 If the tree is not empty, do f(root). 2 recursively apply the function on the left sub-tree 3 recursively apply the function on the right sub-tree Deep-First Search Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

16 Paths finding - search, and applications Looking for all nodes Deep First Search on Graphs On graphs, there can be cycles! Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

17 Paths finding - search, and applications Looking for all nodes Deep First Search on Graphs - Java s t a t i c i n t [ ] num; / / numbering the edges according to the v i s i t i n g or s t a t i c void v i s i t e r ( Graphe g ) { i n t n = g. succ. l e n g t h ; num = new i n t [ n ] ; numordre = 1; for ( i n t x = 0; x < n ; ++x ) num [ x ] = 1; / / 1 = not seen for ( i n t x = 0; x < n ; ++x ) i f (num [ x ] == 1) dfs ( g, x ) ; } s t a t i c void dfs ( Graphe g, i n t x ) { num [ x ] = ++numordre ; for ( L i s t e l s = g. succ [ x ] ; l s!= n u l l ; l s = l s. s u i v a n t ) { i n t y = l s. v a l ; i f (num [ y ] == 1) dfs ( g, y ) ; } } Complexity = O(V + E). Tarjan [1972] Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

18 Paths finding - search, and applications Looking for all nodes Breadth First Search on Graphs Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

19 Paths finding - search, and applications Looking for all nodes Conventions BLANC = not (yet) processed NOIR = has been processed GRIS = being processed f i n a l s t a t i c i n t BLANC = 0, GRIS = 1, NOIR = 2; s t a t i c i n t [ ] couleur ; Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

20 Paths finding - search, and applications Looking for all nodes Breadth First Search on Graphs - Java i n t n = g. succ. l e n g th ; couleur = new i n t [ n ] ; FIFO f = new FIFO ( n ) ; for ( i n t x = 0; x < n ; ++x ) couleur [ x ] = BLANC; for ( i n t x = 0; x < n ; ++x ) i f ( couleur [ x ] == BLANC) { FIFO. a j o u t e r ( f, x ) ; couleur [ x ] = GRIS ; bfs ( g, f ) ; } s t a t i c void bfs ( Graphe g, FIFO f ) { while (! f. vide ( ) ) { i n t x = FIFO. supprimer ( f ) ; for ( L i s t e l s = g. succ [ x ] ; l s!= n u l l ; l s = l s. s u i v a n t ) { i n t y = l s. v a l ; i f ( couleur [ y ] == BLANC) { FIFO. a j o u t e r ( f, y ) ; couleur [ y ] = GRIS ; } couleur [ x ] = NOIR ; } } } Complexity? Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

21 Paths finding - search, and applications Looking for all nodes Application : Labyrinth Finds exit, ie a path from d to s? } couleur [ d ] = GRIS ; i f ( d == s ) return new L i s t e ( d, n u l l ) ; for ( L i s t e l s = g. succ [ d ] ; l s!= n u l l ; l s = l s. s u i v a n t ) { i n t x = l s. v a l ; i f (num [ x ] == BLANC) { L i s t e r = chemin ( g, x, s ) ; i f ( r!= n u l l ) return new L i s t e ( d, r ) ; } } return n u l l ; Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

22 Paths finding - search, and applications Connexity Definition - Connex Component Connex component A connex component is a maximal set of connected vertices. Exercise : Algorithm for computing CCs. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

23 Paths finding - search, and applications Connexity Definition - Connex Graph Connex graph A graph is connex if it has only one connex component. Exercise : Algorithm that tests the connexity of a graph. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

24 Paths finding - search, and applications Topological Sort Other Algorithms There exists variants of DFS algo to : Compute Cover Forets (in bold in the picture) : Detect Cycles. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

25 Algorithms specific to oriented graphs 1 Some generalities on graphs 2 Paths finding - search, and applications 3 Algorithms specific to oriented graphs 4 More difficult problems on graphs 5 NP - completeness 6 Docs Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

26 Algorithms specific to oriented graphs Topologic sort of a DAG Give a list of vertices in an order compatible with the DAG order : v i < v j v j v i (total order from a partial header). Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

27 Algorithms specific to oriented graphs Topologic sort of a DAG - Example If we know an entrant node, a dfs gives the result. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

28 Algorithms specific to oriented graphs Topologic sort of a DAG - implementation f i n a l s t a t i c i n t BLANC = 0, GRIS = 1, NOIR = 2; s t a t i c L i s t e afaire ( Graphe g, i n t x ) { i n t n = g. succ. l e n g t h ; i n t [ ] couleur = new i n t [ n ] ; for ( i n t x = 0; x < n ; ++x ) couleur [ x ] = BLANC; return dfs ( g, x, n u l l, couleur ) ; } s t a t i c L i s t e dfs ( Graphe g, i n t x, L i s t e r, i n t [ ] couleur ) { couleur [ x ] = GRIS ; for ( L i s t e l s = g. succ [ x ] ; l s!= n u l l ; l s = l s. s u i v a n t ) { i n t y = l s. v a l ; i f ( couleur [ y ] == BLANC) r = dfs ( g, y, r, couleur ) ; } return new L i s t e ( x, r ) ; } dfs here returns a list. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

29 Algorithms specific to oriented graphs Transitive closure Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

30 Algorithms specific to oriented graphs Algorithm for Transitive closure - 1 If E is the adjacency matrix : E + = E + E E n 1. An algorithm in O(n 4 ). Is it possible to have a better complexity? Yes (Warshall, in O(n 3 ). Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

31 Algorithms specific to oriented graphs Definition of oriented valuated graph Valuated graph It s a graph with a function d : V V N called distance. G is represented by an adjacency matrix with integer values, or + if there is no edge. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

32 Algorithms specific to oriented graphs The shortest path problem Given G, find a path whose sum of distances is minimal! more variants on Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

33 Algorithms specific to oriented graphs Multiple Source Shortest Path with dynamic programming -1 Between all the vertices! Floyd-Warshall algorithm Principle : Let d k x,y be the minimal distance between x and y passing through vertices whose number is < k. Then d 0 x,y = d x,y and d k x,y = min{d k 1 x,y, d k 1 x,k + dk 1 k,y } Store all these temporary distances. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

34 Algorithms specific to oriented graphs Multiple Source Shortest Path with dynamic programming -2 s t a t i c Graphe pluscourtschemins ( Graphe g ) { i n t n = g. d. l e n g t h ; Graphe gplus = copiegraphe ( g ) ; for ( i n t k = 0; k < n ; ++k ) for ( i n t x = 0; x < n ; ++x ) for ( i n t y = 0; y < n ; ++y ) gplus. d [ x ] [ y ] = Math. min ( gplus. d [ x ] [ y ], gplus. d [ x ] [ k ] + gplus. d [ k ] [ y ] ) ; return gplus ; } Complexity O(n 3 ), space O(n 2 ). Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

35 Algorithms specific to oriented graphs Simple Source Shortest Path All d are > 0 : Dijkstra, 1959 Negative distances : Bellman Ford 1960 Routing algorithms (Bellman uses only local information). Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

36 More difficult problems on graphs 1 Some generalities on graphs 2 Paths finding - search, and applications 3 Algorithms specific to oriented graphs 4 More difficult problems on graphs 5 NP - completeness 6 Docs Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

37 More difficult problems on graphs Eulerian and Hamiltonian paths Eulerian/Hamiltonian Path An Eulerian path : each edge is seen only once. An Hamiltonien path : each node... Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

38 More difficult problems on graphs Hamiltonian Tour, one solution Extend a path until not possible Backtrack one time, and try with another neighbour And so on. Worst case complexity time : exponential in n Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

39 More difficult problems on graphs Application 1 : Knight s Tour Is it possible to make a Knight s tour? «The knight is placed on the empty board and, moving according to the rules of chess, must visit each square exactly once.» A variant of the Hamiltonian tour that can be solved in polynomial time. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

40 More difficult problems on graphs The Travelling Salesman Problem with valuated (non oriented) graph. Find an hamiltonian cycle of minimum weight Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

41 More difficult problems on graphs Travelling Salesman Problem - 2 A (non-polynomial) algorithm for this problem : Transform into a linear programming problem with integers Solve it with the simplex! This algorithm is exponential in the size of the graph Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

42 More difficult problems on graphs Graph Coloring Problem Application to the register allocation in compilers. The sudoku problem (9-coloring of a 81-vertices graph) Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

43 More difficult problems on graphs Graph Coloring Problem - 2 Are you able to design a polynomial algorithm? Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

44 More difficult problems on graphs Graph Coloring Problem - 3 We do not know any polynomial algorithm for this problem. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

45 NP - completeness 1 Some generalities on graphs 2 Paths finding - search, and applications 3 Algorithms specific to oriented graphs 4 More difficult problems on graphs 5 NP - completeness 6 Docs Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

46 NP - completeness Complexity The complexity of an implementation / an algorithm is linked to Turing machines : The number of steps in the execution of a TM gives the complexity in time, The number of seen squares in the execution of a TM gives the complexity in space. We can replace the TM by a C implementation. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

47 NP - completeness P Problems P A problem (or a language) belongs to P if there exists a deterministic Turing Machine that gives the result in polynomial time. there is a polynom p such that for all entry x, the Turing machine stops (with the good result) in less than p( x ) steps. Example : L = a n b n c n, the TM of the previous course checks any a i b i c i in O(n) steps Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

48 NP - completeness NP Problems NP A problem (or a language) belongs to NP if there exists a deterministic Turing Machine that checks the result in polynomial time. Example : there exists a TM that is able to check if a given path is an hamiltonien tour, in polynomial time. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

49 NP - completeness NP vs P - some facts If a problem belongs to NP, then there exists an exponential brute-force deterministic algorithm to solve it (easy to prove) P NP (trivial) P = NP? P NP? Open question! Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

50 NP - completeness NP-complete problems - 1 Given a problem : if we find a polynomial algorithm P if we find a polynomial time checking algorithm NP, but we want to know more : it it really hard to solve it? is it worth looking for a better algorithm? Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

51 NP - completeness NP-complete problems - 2 The key notion is the notion of polynomial reduction Polynomial reduction of problems Given two problems P 1 and P 2, P 1 reduces polynomially to P 2 if there exists an f such that : f is polynomial x 1 solution of P 1 iff f(x 1 ) solution of P 2. Example : Hamiltonian circuit is polynomially reductible to TSP. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

52 NP - completeness NP-complete problems - 3 Reminder : we want to characterise the most difficult problems in NP. NP-complete A problem is said to be NP-complete if : It belongs to NP All other problems in NP reduce polynomially to it. In other words, if we solve any NP-complete in polynomial time, we have proved P = NP (and we become rich) Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

53 NP - completeness NP-complete problems - 4 Problem : how to prove that a problem is NP-complete? The very first one is hard to prove Then, we use the following result : If two problems / languages L 1 and L 2 belong to NP and L 1 is NP-complete, and L 1 reduces polynomially to L 2, then L 2 is NP-complete. Pick one NP complete problem and try to reduce it to your problem. Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

54 NP - completeness NP-complete problems - The historical example SATISFIABILITY is NP-complete (Cook, 1971) SAT reduces polynomially to 3SAT 3SAT reduces polynomially to Vertex Cover Vertex Cover reduces polynomially to Hamiltonian circuit So Hamiltonian circuit, then TSP are also NP-complete problems Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

55 NP - completeness Some NP-complete (common) problems Some classical examples : Hamiltonian cycle and TSP Graph Coloring Problem Vertex cover Knapsack (integer) Flow shop problem The book : Computers and Intractability : A Guide to the Theory of NP-Completeness. A short list on the french webpage : NP-complets Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

56 NP - completeness Conclusion - 1 Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

57 NP - completeness Conclusion - 2 Knowing that a given problem is NP complete is just the beginning : handle less general classes of inputs compute less precise solutions Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

58 Docs 1 Some generalities on graphs 2 Paths finding - search, and applications 3 Algorithms specific to oriented graphs 4 More difficult problems on graphs 5 NP - completeness 6 Docs Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

59 Docs More docs on graphs In french : graphes/cours/cours_graphes.pdf or type cours de Théorie des graphes in Google In english : an interactive tutorial here : utm.edu/cgi-bin/caldwell/tutor/graph/intro. (en) The theorical book Graph Theory, R. Diestel (electronic version : (en) e-book for algorithms : http: //code.google.com/p/graph-theory-algorithms-book/ Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

60 Docs More docs on complexity The Book : computers and intractability, a guide to the theory of NP completeness, by Garey/Johnson Laure Gonnord (Lille1/Polytech) Informatique Fondamentale IMA S8 March/April / 60

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