Abundant Extensions -- Three Problems on Well-Partitioned Chordal Graphs -- Distributed Distance-r Covering Problems on Sparse High-Girth Graphs -- Reconfiguration of Connected Graph Partitions via Recombination -- Algorithms for Energy Conservation in Heterogeneous Data Centers -- On Vertex-Weighted Graph Realizations -- On the Role of 3's for the 1-2-3 Conjecture -- Upper Tail Analysis of Bucket Sort and Random Tries -- Throughput Scheduling with Equal Additive Laxity -- Fragile Complexity of Adaptive Algorithms -- FPT and Kernelization Algorithms for the Induced Tree Problem -- A Tight Lower Bound for Edge-Disjoint Paths on Planar DAGs -- Upper Dominating Set: Tight Algorithms for Pathwidth and Sub-Exponential Approximation -- A Multistage View on 2-Satisfiability -- The Weisfeiler-Leman Algorithm and Recognition of Graph Properties -- The Parameterized Suffix Tray -- Exploring the Gap Between Treedepth and Vertex Cover Through Vertex Integrity -- Covering a Set of Line Segments with a Few Squares -- Circumventing Connectivity for Kernelization -- Online and Approximate Network Construction from Bounded Connectivity Constraints -- Globally Rigid Augmentation of Minimally Rigid Graphs in \(R^2\) -- Extending Partial Representations of Rectangular Duals with Given Contact Orientations -- Can Local Optimality be Used for Efficient Data Reduction -- Colouring Graphs of Bounded Diameter in the Absence of Small Cycles -- Online Two-Dimensional Vector Packing with Advice -- Temporal Matching on Geometric Graph Data.

This book constitutes the refereed conference proceedings of the 12th International Conference on Algorithms and Complexity, CIAC 2019, held as a virtual event, in May 2021. The 28 full papers presented together with one invited lecture and 2 two abstracts of invited lectures were carefully reviewed and selected from 78 submissions. The International Conference on Algorithms and Complexity is intended to provide a forum for researchers working in all aspects of computational complexity and the use, design, analysis and experimentation of efficient algorithms and data structures. The papers present original research in the theory and applications of algorithms and computational complexity. Due to the Corona pandemic the conference was held virtually.