The 26th International Symposium on Algorithms and Computation (ISAAC 2015) was held in Nagoya, Japan, December 9-11, 2015. The program committee received 180 high-quality submissions, and 65 were accepted for presentation. This special issue gathers a selection of five of these accepted papers, which went through the standard refereeing process of Algorithmica. In the paper by Aline Saettler, Eduardo Laber and Ferdinando Cicalese, the authors characterize the best possible trade-off achievable

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... when optimizing the construction of a decision tree with respect to both the worst and the expected costs, given a probability distribution on the input. For every ρ > 0, they show that there is a decision tree with worst-case cost of at most (1 + ρ)OPT W + 1 and expected cost at most 1/(1 − e −ρ )OPT E , where OPT W and OPT E denote respectively the minimum worst-case cost and the minimum expected cost of a decision tree for the given instance. They also present an infinite family of instances for which this is essentially the best possible trade-off. The paper by Sumedh Tirodkar and Sundar Vishwanathan is on the approximability of the minimum rainbow subgraph (MRS) problem, which has applications in computational biology. Given an n-vertex edge-colored undirected graph, the goal is to find a subgraph on a minimum number of vertices which has one induced edge of each color. By considering a colored version of the well-known k-densest subgraph B Khaled Elbassioni