A faster computation of all the best swap edges of a shortest paths tree

We consider a 2-edge connected, non-negatively weighted graph G, with n nodes and m edges, and a single-source shortest paths tree (SPT) of G rooted at an arbitrary node. If an edge of the SPT is temporarily removed, a widely recognized approach to reconnect the nodes disconnected from the root consists of joining the two resulting subtrees by means of a single non-tree edge, called a swap edge. This allows to reduce consistently the set-up and computational costs which are incurred if we instead rebuild a new optimal SPT from scratch. In the past, several optimality criteria have been considered to select a best possible swap edge, and here we restrict our attention to arguably the two most significant measures: the minimization of either the maximum or the average distance between the root and the disconnected nodes. For the former criteria, we present an O(m logα(m,n)) time algorithm to find a best swap edge for every edge of the SPT, thus improving onto the previous O(m logn) time algorithm (B. Gfeller, ESA’08). Concerning the latter criteria, we provide an O(m + n logn) time algorithm for the special but important case where G is unweighted, which compares favorably with the O(m+nα(n,n)log2n) time bound that one would get by using the fastest algorithm known for the weighted case – once this is suitably adapted to the unweighted case.