The reachability problem asks to decide if there exists a path from one vertex to another in a digraph. In a grid digraph, the vertices are the points of a two-dimensional square grid, and an edge can occur between a ...
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The reachability problem asks to decide if there exists a path from one vertex to another in a digraph. In a grid digraph, the vertices are the points of a two-dimensional square grid, and an edge can occur between a vertex and its immediate horizontal and vertical neighbors *** and Doerr (CCCG'11) presented the first simultaneous time-space bound for reachability in grid digraphs by solving the problem in polynomial time and O(n(1/2+E)) space. In 2018, the space complexity was improved to O(n(1/3)) by Ashida and Nakagawa (SoCG'18).In this paper, we show that there exists a polynomial-time algorithm that uses O(n(1/4+E)) space to solve the reachability problem in a grid digraph containing n vertices. We define and construct a new separator-like device called pseudoseparator to develop a divide-and-conquer algorithm. This algorithm works in a space-efficient manner to solve reachability.
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