PERFECT MATCHINGS IN O(n log n) TIME IN REGULAR BIPARTITE GRAPHS

In this paper we consider the well-studied problem of finding a perfect matching in a d-regular bipartite graph on 2n nodes with m = nd edges. The best known algorithm for general bipartite graphs (due to Hopcroft and Karp) takes time O(m root n). In regular bipartite graphs, however, a matching is known to be computable in O(m) time (due to Cole, Ost, and Schirra). In a recent line of work by Goel, Kapralov, and Khanna the O(m) time bound was improved first to (O) over tilde (min{m, n(2.5)/d}) and then to (O) over tilde (min{m, n(2)/d}). In this paper, we give a randomized algorithm that finds a perfect matching in a d-regular graph and runs in O(n log n) time (both in expectation and with high probability). The algorithm performs an appropriately truncated alternating random walk to successively find augmenting paths. Our algorithm may be viewed as using adaptive uniform sampling, and is thus able to bypass the limitations of (nonadaptive) uniform sampling established in earlier work. Our techniques also give an algorithm that successively finds a matching in the support of a doubly stochastic matrix in expected time O(n log(2) n), with O(m) preprocessing time; this gives a simple O(m+mnlog(2) n) time algorithm for finding the Birkhoff-von Neumann decomposition of a doubly stochastic matrix. We show that randomization is crucial for obtaining o(nd) time algorithms by establishing an Omega(nd) lower bound for deterministic algorithms. We also show that there does not exist a randomized algorithm that finds a matching in a regular bipartite multigraph and takes o(n log n) time with high probability.