Title: Distributed Exact Shortest Paths in Sublinear Time,
Abstract: The distributed single-source shortest paths problem is one of the most
fundamental and central problems in the message-passing distributed computing.
Classical Bellman-Ford algorithm solves it in $O(n)$ time, where $n$ is the
number of vertices in the input graph $G$. Peleg and Rubinovich (FOCS'99)
showed a lower bound of $\tilde{\Omega}(D + \sqrt{n})$ for this problem, where
$D$ is the hop-diameter of $G$.
Whether or not this problem can be solved in $o(n)$ time when $D$ is
relatively small is a major notorious open question. Despite intensive research
\cite{LP13,N14,HKN15,EN16,BKKL16} that yielded near-optimal algorithms for the
approximate variant of this problem, no progress was reported for the original
problem.
In this paper we answer this question in the affirmative. We devise an
algorithm that requires $O((n \log n)^{5/6})$ time, for $D = O(\sqrt{n \log
n})$, and $O(D^{1/3} \cdot (n \log n)^{2/3})$ time, for larger $D$. This
running time is sublinear in $n$ in almost the entire range of parameters,
specifically, for $D = o(n/\log^2 n)$. For the all-pairs shortest paths
problem, our algorithm requires $O(n^{5/3} \log^{2/3} n)$ time, regardless of
the value of $D$.
We also devise the first algorithm with non-trivial complexity guarantees for
computing exact shortest paths in the multipass semi-streaming model of
computation.
From the technical viewpoint, our algorithm computes a hopset $G"$ of a
skeleton graph $G'$ of $G$ without first computing $G'$ itself. We then conduct
a Bellman-Ford exploration in $G' \cup G"$, while computing the required edges
of $G'$ on the fly. As a result, our algorithm computes exactly those edges of
$G'$ that it really needs, rather than computing approximately the entire $G'$.