 
Summary: 859
On External Memory GraphTraversal
Adam L. Buchsbaum* Michael Goldwasser t Suresh Venkatasubramanian*
Jeffery R. Westbrook*
Abstract
We describe a new external memory data structure, the buffered
repository tree,and use it to provide the first nontrivial external
memory algorithm for directed breadthfirst search (BFS) and an
improved external algorithm fordirected depthfirst search. Wealso
demonstrate the equivalence of various formulations of external
undirected BFS, and we use these to give the first I/Ooptimal BFS
algorithm for undirected trees.
1 Introduction
We use the standard 110 model [1], which counts disk ac
cesses incurred by an algorithm, using the following param
eters: M is the memory size, B is the block size, and we as
sume that B < M/2. Define sort(N) = O( N lOgM/B N),
the number of 11Os needed to sort N items, and scan(N) =
[N/B], the number of I/Os needed to transfer N contiguous
items between disk and internal memory.
