Linear-Processor NC Algorithms for Planar Directed Graphs I: Strongly Connected Components

Abstract

Finding strongly connected components is a fundamental step in many algorithms for directed graphs. In sequential computation this problem has an optimal linear-time algorithm. In parallel computation, however, it remains an open problem whether polylogarithmic time and a linear number of processors are sufficient for computing the strongly connected components on a parallel random-access machine. This paper provides the first nontrivial partial solution to the open problem: for a planar directed graph of size n the strongly connected components can be computed deterministically in $O(\log ^3 n)$ time with ${n / {\log n}}$ processors. The algorithm runs on a parallel random-access machine that allows concurrent reads and concurrent writes in its shared memory and, in case of a write conflict, permits an arbitrary processor to succeed.