论文标题
通过定向扩张器和拥塞平衡的确定性降低可及性,SCC和最短路径
Deterministic Decremental Reachability, SCC, and Shortest Paths via Directed Expanders and Congestion Balancing
论文作者
论文摘要
令$ g =(v,e,w)$为加权的,受对抗边缘删除序列的约束。在减少单源可达性问题(SSR)中,我们获得了一个固定源$ S $,目标是维护一个数据结构,该数据结构可以回答V $中任何$ v \ for v $中的任何$ v \。在更通用的单源最短路径(SSSP)问题中,目标是将大约最短的路径返回到$ v $,而在SCC问题中,目标是维持$ g $的牢固连接的组件,并回答每个组件中的路径查询。在过去的二十年中,所有这些问题已经进行了非常积极的研究,但是所有快速算法都是随机的,更重要的是,只有在假设较弱的模型中,它们只能回答路径查询:他们假定遗忘的对手,这不是自适应的,并且必须提前修复更新顺序。该假设显着限制了这些数据结构的使用,最著名的是阻止它们用作静态算法中的子例程。众所周知,在自适应环境中,所有上述问题都是困难的。实际上,最先进的东西仍然是均匀而卑鄙的树,它的历史可以追溯到1981年,并实现了总更新时间$ O(MN)$。我们提出了第一个突破此障碍的算法: 1)确定性减少SSR/SCC,具有总更新时间$ Mn^{2/3 + O(1)} $ 2)确定性减少SSSP,总更新时间$ n^{2+2/3+o(1)} $。 为了实现这些结果,我们开发了两种具有动态图的更广泛兴趣的通用技术:1)将基于扩展器的工具概括为动态有向图的概括,以及2)一种我们称为拥塞平衡的技术,并为对抗性缺失下的流量提供了一种新方法。使用第二种技术,我们提供了第一种近乎最佳的算法,以减少两部分匹配。
Let $G = (V,E,w)$ be a weighted, digraph subject to a sequence of adversarial edge deletions. In the decremental single-source reachability problem (SSR), we are given a fixed source $s$ and the goal is to maintain a data structure that can answer path-queries $s \rightarrowtail v$ for any $v \in V$. In the more general single-source shortest paths (SSSP) problem the goal is to return an approximate shortest path to $v$, and in the SCC problem the goal is to maintain strongly connected components of $G$ and to answer path queries within each component. All of these problems have been very actively studied over the past two decades, but all the fast algorithms are randomized and, more significantly, they can only answer path queries if they assume a weaker model: they assume an oblivious adversary which is not adaptive and must fix the update sequence in advance. This assumption significantly limits the use of these data structures, most notably preventing them from being used as subroutines in static algorithms. All the above problems are notoriously difficult in the adaptive setting. In fact, the state-of-the-art is still the Even and Shiloach tree, which dates back all the way to 1981 and achieves total update time $O(mn)$. We present the first algorithms to break through this barrier: 1) deterministic decremental SSR/SCC with total update time $mn^{2/3 + o(1)}$ 2) deterministic decremental SSSP with total update time $n^{2+2/3+o(1)}$. To achieve these results, we develop two general techniques of broader interest for working with dynamic graphs: 1) a generalization of expander-based tools to dynamic directed graphs, and 2) a technique that we call congestion balancing and which provides a new method for maintaining flow under adversarial deletions. Using the second technique, we provide the first near-optimal algorithm for decremental bipartite matching.