Fully Dynamic Shortest Path Reporting Against an Adaptive Adversary
Algebraic data structures are the main subroutine for maintaining distances in fully dynamic graphs in subquadratic time. However, these dynamic algebraic algorithms generally cannot maintain the shortest paths, especially against adaptive adversaries. We present the first fully dynamic algorithm that maintains the shortest paths against an adaptive adversary in subquadratic update time. This is obtained via a combinatorial reduction that allows reconstructing the shortest paths with only a few distance estimates. Using this reduction, we obtain the following: On weighted directed graphs with real edge weights in $[1,W]$, we can maintain $(1+\epsilon)$ approximate shortest paths in $\tilde{O}(n^{1.816}\epsilon^{-2} \log W)$ update and $\tilde{O}(n^{1.741} \epsilon^{-2} \log W)$ query time. This improves upon the approximate distance data structures from [v.d.Brand, Nanongkai, FOCS'19], which only returned a distance estimate, by matching their complexity and returning an approximate shortest path. On unweighted directed graphs, we can maintain exact shortest paths in $\tilde{O}(n^{1.823})$ update and $\tilde{O}(n^{1.747})$ query time. This improves upon [Bergamaschi, Henzinger, P.Gutenberg, V.Williams, Wein, SODA'21] who could report the path only against oblivious adversaries. We improve both their update and query time while also handling adaptive adversaries. On unweighted undirected graphs, our reduction holds not just against adaptive adversaries but is also deterministic. We maintain a $(1+\epsilon)$-approximate $st$-shortest path in $O(n^{1.529} / \epsilon^2)$ time per update, and $(1+\epsilon)$-approximate single source shortest paths in $O(n^{1.764} / \epsilon^2)$ time per update. Previous deterministic results by [v.d.Brand, Nazari, Forster, FOCS'22] could only maintain distance estimates but no paths.