Journal of Experimental Algorithmics (JEA)
WEA'07 Proceedings of the 6th international conference on Experimental algorithms
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The dynamic trees problem is that of maintaining a forest that changes over time through edge insertions and deletions. We can associate data with vertices or edges and manipulate this data, individually or in bulk, with operations that deal with whole paths or trees. Efficient solutions to this problem have numerous applications, particularly in algorithms for network flows and dynamic graphs in general. Several data structures capable of logarithmic-time dynamic tree operations have been proposed. The first was Sleator and Tarjan's ST-tree, which represents a partition of the tree into paths. Although reasonably fast in practice, adapting ST-trees to different applications is nontrivial. Frederickson's topology trees, Alstrup et al.'s top trees, and Acar et al.'s RC-trees are based on tree contractions: they progressively combine vertices or edges to obtain a hierarchical representation of the tree. This approach is more flexible in theory, but all known implementations assume the trees have bounded degree; arbitrary trees are supported only after ternarization. This thesis shows how these two approaches can be combined (with very little overhead) to produce a data structure that is at least as generic as any other, very easy to adapt, and as practical as ST-trees. It can be seen as a self-adjusting implementation of top trees and provides a logarithmic bound per operation in the amortized sense. We also discuss a pure contraction-based implementation of top trees, which is more involved but guarantees a logarithmic bound in the worst case. Finally, an experimental evaluation of these two data structures, including a comparison with previous methods, is presented.