Min st -Cut Oracle for Planar Graphs with Near-Linear Preprocessing Time
Glencora Borradaile, Piotr Sankowski, Christian Wulff‐Nilsen · ACM Transactions on Algorithms · 2015
For an undirected n -vertex planar graph G with nonnegative edge weights, we consider the following type of query: given two vertices s and t in G , what is the weight of a min st -cut in G ? We show how to answer such queries in constant time with O ( n log 4 n ) preprocessing time and O ( n log n ) space. We use a Gomory-Hu tree to represent all the pairwise min cuts implicitly. Previously, no subquadratic time algorithm was known for this problem. Since all-pairs min cut and the minimum-cycle basis are dual problems in planar graphs, we also obtain an implicit representation of a minimum-cycle basis in O ( n log 4 n ) time and O ( n log n ) space. Additionally, an explicit representation can be obtained in O ( C ) time and space where C is the size of the basis. These results require that shortest paths are unique. This can be guaranteed either by using randomization without overhead or deterministically with an additional log 2 n factor in the preprocessing times.