Subquadratic Algorithms for 3SUM

  • Authors:
  • Ilya Baran;Erik D. Demaine;Mihai Pǎtraşcu

  • Affiliations:
  • MIT Computer Science and Artificial Intelligence Laboratory, 32 Vassar Street, 02139, Cambridge, MA, USA;MIT Computer Science and Artificial Intelligence Laboratory, 32 Vassar Street, 02139, Cambridge, MA, USA;MIT Computer Science and Artificial Intelligence Laboratory, 32 Vassar Street, 02139, Cambridge, MA, USA

  • Venue:
  • Algorithmica
  • Year:
  • 2008

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Abstract

We obtain subquadratic algorithms for 3SUM on integers and rationals in several models. On a standard word RAM with w-bit words, we obtain a running time of $O(n^{2}/\max\{\frac{w}{\lg^{2}w},\frac{\lg^{2}n}{(\lg\lg n)^{2}}\})$. In the circuit RAM with one nonstandard AC0 operation, we obtain $O(n^{2}/\frac{w^{2}}{\lg^{2}w})$. In external memory, we achieve O(n2/(MB)), even under the standard assumption of data indivisibility. Cache-obliviously, we obtain a running time of $O(n^{2}/\frac{MB}{\lg^{2}M})$. In all cases, our speedup is almost quadratic in the “parallelism” the model can afford, which may be the best possible. Our algorithms are Las Vegas randomized; time bounds hold in expectation, and in most cases, with high probability.