943 research outputs found
Exact Distance Oracles for Planar Graphs with Failing Vertices
We consider exact distance oracles for directed weighted planar graphs in the
presence of failing vertices. Given a source vertex , a target vertex
and a set of failed vertices, such an oracle returns the length of a
shortest -to- path that avoids all vertices in . We propose oracles
that can handle any number of failures. More specifically, for a directed
weighted planar graph with vertices, any constant , and for any , we propose an oracle of size
that answers queries in
time. In particular, we show an
-size, -query-time
oracle for any constant . This matches, up to polylogarithmic factors, the
fastest failure-free distance oracles with nearly linear space. For single
vertex failures (), our -size,
-query-time oracle improves over the previously best
known tradeoff of Baswana et al. [SODA 2012] by polynomial factors for , . For multiple failures, no planarity exploiting
results were previously known
Capacitated Dynamic Programming: Faster Knapsack and Graph Algorithms
One of the most fundamental problems in Computer Science is the Knapsack
problem. Given a set of n items with different weights and values, it asks to
pick the most valuable subset whose total weight is below a capacity threshold
T. Despite its wide applicability in various areas in Computer Science,
Operations Research, and Finance, the best known running time for the problem
is O(Tn). The main result of our work is an improved algorithm running in time
O(TD), where D is the number of distinct weights. Previously, faster runtimes
for Knapsack were only possible when both weights and values are bounded by M
and V respectively, running in time O(nMV) [Pisinger'99]. In comparison, our
algorithm implies a bound of O(nM^2) without any dependence on V, or O(nV^2)
without any dependence on M. Additionally, for the unbounded Knapsack problem,
we provide an algorithm running in time O(M^2) or O(V^2). Both our algorithms
match recent conditional lower bounds shown for the Knapsack problem [Cygan et
al'17, K\"unnemann et al'17].
We also initiate a systematic study of general capacitated dynamic
programming, of which Knapsack is a core problem. This problem asks to compute
the maximum weight path of length k in an edge- or node-weighted directed
acyclic graph. In a graph with m edges, these problems are solvable by dynamic
programming in time O(km), and we explore under which conditions the dependence
on k can be eliminated. We identify large classes of graphs where this is
possible and apply our results to obtain linear time algorithms for the problem
of k-sparse Delta-separated sequences. The main technical innovation behind our
results is identifying and exploiting concavity that appears in relaxations and
subproblems of the tasks we consider
A graph-based mathematical morphology reader
This survey paper aims at providing a "literary" anthology of mathematical
morphology on graphs. It describes in the English language many ideas stemming
from a large number of different papers, hence providing a unified view of an
active and diverse field of research
Distribution-Aware Compressed Full-Text Indexes
Peer reviewe
Distribution-aware compressed full-text indexes
In this paper we address the problem of building a compressed self-index that, given a distribution for the pattern queries and a bound on the space occupancy, minimizes the expected query time within that index space bound. We solve this problem by exploiting a reduction to the problem of finding a minimum weight K-link path in a properly designed Directed Acyclic Graph. Interestingly enough, our solution can be used with any compressed index based on the Burrows-Wheeler transform. Our experiments compare this optimal strategy with several other known approaches, showing its effectiveness in practice
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