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1 Advanced Algorithms All-pairs SPs DP algorithm Floyd-Warshall alg.
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2 APSP Problem Given a directed graph G = (V, E), weight function w : E → R, |V| = n determine the length of the shortest path between all pairs of vertices in G. In other words, we want to create an n × n matrix of shortest- path distances δ(u, v). Here we assume that there are no cycles with zero or negative cost. Could run BELLMAN-FORD once from each vertex: O(V 2 E) - which is O(V 4 ) if the graph is dense (E = Ө(V 2 )).
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3 APSP (cont.) If no negative-weight edges, could run Dijkstra.s algorithm once from each vertex: O(V E lg V) with binary heap - O(V 3 lg V) if dense, O(V 2 lg V + V E) with Fibonacci heap - O(V 3 ) if dense. We will see how to do in O(V 3 ) in all cases, with no fancy data structure.
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4 Terminologies G = (V, E) where V are numbered 1, 2,.. n=|V| Input :, n n matrix Output:, n n matrix
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5 Dynamic Programming Approach Optimal substructure property holds Define subproblem: : minimum weight of any path from vertex i to j that contains at most m edges. Recursive solution Base case : m = 0
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6 Recursive Solution cont. Goal: Example:
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7 Build DP-table Compute in order = W Go from
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8 Pseudo-code cont. Time complexity: Extend: Slow-Apsp: Can we do better ?
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9 Matrix Multiplication Extend procedure same as matrix multiplication L’ = L W ! = L (m) W Goal: compute W (n-1) Only need to compute,,, …, W 2 W 1 W (n-1) W 4
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10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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11 Faster Algorithm Time complexity:
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12 Floyd-Warshall Algorithm Structure of a shortest path
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13 Structure of a shortest path (cont.)
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14 Recursive solution Recursive solution: : the weight of shortest path from vertex i to j where all intermediate vertices are from set {1, 2, …, k}. Goal: To compute = D (n)
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15 Pseudo-code Time complexity:
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16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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17 Constructing a shortest Path Compute a sequence of matrices Π (0), Π (1), Π (2),…, Π (n), where Π= Π (n).
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18 Construct a shortest path Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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19 Thank you.
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