16-Greedy-Algorithms

16-Greedy-Algorithms - Algorithms LECTURE 16 Greedy...

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Algorithms L16.1 Professor Ashok Subramanian L ECTURE 16 Greedy Algorithms (and Graphs) Graph representation Minimum spanning trees Optimal substructure Greedy choice Prim’s greedy MST algorithm Algorithms
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Algorithms L16.2 Graphs (review) Definition. A directed graph ( digraph ) G = ( V , E ) is an ordered pair consisting of a set V of vertices (singular: vertex ), a set E V × V of edges . In an undirected graph G = ( V , E ) , the edge set E consists of unordered pairs of vertices. In either case, we have | E | = O ( V ) . Moreover, (Review CLRS, Appendix B.)
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Algorithms L16.3 Adjacency-matrix representation The adjacency matrix of a graph G = ( V , E ) , where V = {1, 2, …, n } , is the matrix A [1 . . n , 1 . . n ] given by A [ i , j ] = 1 if ( i , j ) E , 0 if ( i , j ) E .
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Algorithms L16.4 Adjacency-matrix representation The adjacency matrix of a graph G = ( V , E ) , where V = {1, 2, …, n } , is the matrix A [1 . . n , 1 . . n ] given by A [ i , j ] = 1 if ( i , j ) E , 0 if ( i , j ) E . 2 1 3 4 A 1 2 3 4 1 2 3 4 0 1 1 0 0 0 1 0 0 0 0 0 0 0 1 0 Θ ( V ) storage
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Algorithms L16.5 Adjacency-list representation An adjacency list of a vertex v V is the list Adj [ v ] of vertices adjacent to v . 2 1 3 4 Adj [1] = {2, 3} Adj [2] = {3} Adj [3] = {} Adj [4] = {3}
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Algorithms L16.6 Adjacency-list representation An adjacency list of a vertex v V is the list Adj [ v ] of vertices adjacent to v . 2 1 3 4 Adj [1] = {2, 3} Adj [2] = {3} Adj [3] = {} Adj [4] = {3} For undirected graphs, | Adj [ v ] | = degree ( v ) . For digraphs, | Adj [ v ] | = out-degree ( v ) .
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Algorithms L16.7 Adjacency-list representation An adjacency list of a vertex v V is the list Adj [ v ] of vertices adjacent to v . 2 1 3 4 Adj [1] = {2, 3} Adj [2] = {3} Adj [3] = {} Adj [4] = {3} For undirected graphs, | Adj [ v ] | = degree ( v ) . For digraphs, | Adj [ v ] | = out-degree ( v ) . Handshaking Lemma: = 2 | E | for undirected
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Algorithms L16.8 Minimum spanning trees Input: A connected, undirected graph G = ( V , E ) with weight function w : E R . For simplicity, assume that all edge weights are distinct. (CLRS covers the general case.)
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Algorithms L16.9 Minimum spanning trees Input: A connected, undirected graph G = ( V , E ) with weight function w : E R . For simplicity, assume that all edge weights are
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This note was uploaded on 04/29/2011 for the course IT 201 taught by Professor K.v.arya during the Spring '11 term at IIT Kanpur.

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16-Greedy-Algorithms - Algorithms LECTURE 16 Greedy...

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