Internet Engineering Czesław Smutnicki Discrete Mathematics – Graphs and Algorithms

CONTENTS Fundamental notions Shortest path Shortest path from the source; non- negative weights; Dijkstra’s algorithm; Shortest path between each pair of nodes; Floyd-Warshall algorithm; Shortest path from the source; any weights; Bellman-Ford algorithm; Longest path Longest path in acyclic graph; Bellman algorithm; Longest path in any graph; Bellman- Ford algorithm; Minimal spanning tree Prime algorithm Kruskal algorithm Maximum flow Definitions and properties Ford-Fulkerson algorithm Dinic algorithm

GRAPHS. BASIC NOTIONS nodes arcs, edges weights graph, digraph, multigraph node degree, isolated node, leaf tree, spanning tree, rooted/unrooted tree path, path length, chain cycle, Hamiltonian cycle, cycle length AoN, AoA representations flow, node divergence, cut, minimal cut data structures for graphs complexity analysis

SHORTEST PATHS IN Graph FROM source. DIJKSTRA’S ALGORITHM 1.function Dijkstra(Graph, source): 2.for each vertex v in Graph: // Initializations 3.dist[v] := infinity ; // Unknown distance function from source to v 4.previous[v] := undefined ; // Previous node in optimal path from source 5.end for ; 6.dist[source] := 0 ; // Distance from source to source 7.Q := the set of all nodes in Graph ; // All nodes in the graph are unoptimized - thus are in Q 8.while Q is not empty: // The main loop 9.u := vertex in Q with smallest dist[] ; 10.if dist[u] = infinity: 11.break ; // all remaining vertices are inaccessible from source 12.fi ; 13.remove u from Q ; 14.for each neighbor v of u: // where v has not yet been removed from Q. 15.alt := dist[u] + dist_between(u, v) ; 16.if alt < dist[v]: // Relax (u,v,a) 17.dist[v] := alt ; 18.previous[v] := u ; 19.fi ; 20.end for ; 21.end while ; 22.return dist[] ; 23.end Dijkstra.

SHORTEST PATHS BETWEEN EACH PAIR OF NODES. FLOYD-WARSHALL ALGORITHM 1./* Assume a function edgeCost(i,j) which returns the cost of the edge from i to j 2.(infinity if there is none). 3.Also assume that n is the number of vertices and edgeCost(i,i) = 0 4.*/ 5. 6.int path[][]; 7./* A 2-dimensional matrix. At each step in the algorithm, path[i][j] is the shortest path 8.from i to j using intermediate vertices (1..k−1). Each path[i][j] is initialized to 9.edgeCost(i,j). 10.*/ procedure FloydWarshall () 13.for k := 1 to n 14.for i := 1 to n 15.for j := 1 to n 16.path[i][j] = min ( path[i][j], path[i][k]+path[k][j] );

SHORTEST PATHS BETWEEN EACH PAIR OF NODES. FLOYD-WARSHALL ALGORITHM cont. 17.procedure FloydWarshallWithPathReconstruction () 18.for k := 1 to n 19.for i := 1 to n 20.for j := 1 to n 21.if path[i][k] + path[k][j] < path[i][j] then 22.path[i][j] := path[i][k]+path[k][j]; 23.next[i][j] := k; procedure GetPath (i,j) 26.if path[i][j] equals infinity then 27.return "no path"; 28.int intermediate := next[i][j]; 29.if intermediate equals 'null' then 30.return " "; /* there is an edge from i to j, with no vertices between */ 31.else 32.return GetPath(i,intermediate) + intermediate + GetPath(intermediate,j);

SHORTEST PATHS FROM source. BELLMAN-FORD ALGORITHM 1.procedure BellmanFord(list vertices, list edges, vertex source) 2.// This implementation takes in a graph, represented as lists of vertices 3.// and edges, and modifies the vertices so that their distance and 4.// predecessor attributes store the shortest paths. 5.// Step 1: initialize graph 6.for each vertex v in vertices: 7.if v is source then v.distance := 0 8.else v.distance := infinity 9.v.predecessor := null 10.// Step 2: relax edges repeatedly 11.for i from 1 to size(vertices)-1: 12.for each edge uv in edges: // uv is the edge from u to v 13.u := uv.source 14.v := uv.destination 15.if u.distance + uv.weight < v.distance: 16.v.distance := u.distance + uv.weight 17.v.predecessor := u 18.// Step 3: check for negative-weight cycles 19.for each edge uv in edges: 20.u := uv.source 21.v := uv.destination 22.if u.distance + uv.weight < v.distance: 23.error "Graph contains a negative-weight cycle"

MAXIUMUM FLOW FROM source TO sink. FORD-FULKERSON ALGORITHM 1.class Edge: 2.def __init__(self, u, v, w): 3.self.source = u 4.self.sink = v 5.self.capacity = w 6.def __repr__(self): 7.return str(self.source) + "->" + str(self.sink) + " : " + str(self.capacity) 8.class FlowNetwork(object): 9.def __init__(self): 10.self.adj, self.flow, = {}, {} 11.def add_vertex(self, vertex): 12.self.adj[vertex] = [] 13.def get_edges(self, v): 14.return self.adj[v] 15.def add_edge(self, u, v, w=0): 16.assert(u != v) 17.edge = Edge(u,v,w) 18.redge = Edge(v,u,0) 19.edge.redge = redge 20.redge.redge = edge 21.self.adj[u].append(edge) 22.self.adj[v].append(redge) 23.self.flow[edge] = 0 24.self.flow[redge] = 0 25.

MAXIUMUM FLOW FROM source TO sink. FORD-FULKERSON ALGORITHM. cont 26.def find_path(self, source, sink, path): 27.if source == sink: 28.return path 29.for edge in self.get_edges(source): 30.residual = edge.capacity - self.flow[edge] 31.if residual > 0 and not (edge,residual) in path: 32.result = self.find_path( edge.sink, sink, path + [(edge,residual)] ) 33.if result != None: 34.return result 35.def max_flow(self, source, sink): 36.path = self.find_path(source, sink, []) 37.while path != None: 38.flow = min(res for edge,res in path) 39.for edge,res in path: 40.self.flow[edge] += flow 41.self.flow[edge.redge] -= flow 42.path = self.find_path(source, sink, []) 43.return sum(self.flow[edge] for edge in self.get_edges(source))

Thank you for your attention DISCRETE MATHEMATICS Czesław Smutnicki