We introduce a quantum algorithm design paradigm called combine and conquer, which is a quantum version of the "marriage-before-conquest" technique of Kirkpatrick and Seidel. In a quantum combine-and-conquer algorithm, one performs the essential computation of the combine step of a quantum divide-and-conquer algorithm prior to the conquer step while avoiding recursion. This model is better suited for the quantum setting, due to its non-recursive nature. We show the utility of this approach by providing quantum algorithms for 2D maxima set and convex hull problems for sorted point sets running in Õ(√{nh}) time, w.h.p., where h is the size of the output.
@InProceedings{fukuzawa_et_al:LIPIcs.SoCG.2025.51, author = {Fukuzawa, Shion and Goodrich, Michael T. and Irani, Sandy}, title = {{Quantum Combine and Conquer and Its Applications to Sublinear Quantum Convex Hull and Maxima Set Construction}}, booktitle = {41st International Symposium on Computational Geometry (SoCG 2025)}, pages = {51:1--51:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-370-6}, ISSN = {1868-8969}, year = {2025}, volume = {332}, editor = {Aichholzer, Oswin and Wang, Haitao}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2025.51}, URN = {urn:nbn:de:0030-drops-232035}, doi = {10.4230/LIPIcs.SoCG.2025.51}, annote = {Keywords: quantum computing, computational geometry, divide and conquer, convex hulls, maxima sets} }
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