COMPUTING THE ALL-PAIRS LONGEST CHAINS IN THE PLANE

Abstract

Many problems on sequences and on special kinds of graphs involve the computation of longest chains passing points in the plane. Given a set S of n points in the plane, we consider the problem of computing the matrix of longest chain lengths between all pairs of points in S, and the matrix of “parent” pointers that describes the n longest chain trees. We present a simple sequential algorithm for computing these matrices. Our algorithm runs in O(n2) time, and hence is optimal. We also present a rather involved parallel algorithm that computes these matrices in O((log n)2) time using O(n2/log n) processors in the CREW PRAM model. These matrices enable us to report, in O(1) time, the length of a longest chain between any two points in S by using one processor, and the actual chain by using k processors, where k is the number of points of S on that chain. The space complexity of the algorithms is O(n2).