Breaking the Cubic Barrier for All-Pairs Max-Flow: Gomory-Hu Tree in Nearly Quadratic Time
In 1961, Gomory and Hu showed that the All-Pairs Max-Flow problem of computing the max-flow between all (n/2) pairs of vertices in an undirected graph can be solved using only n-1 calls to any (single-pair) max-flow algorithm. Even assuming a linear-time max-flow algorithm, this yields a running time of O(mn), which is O(n3) when m = T(n2). While subsequent work has improved this bound for various special graph classes, no subcubic-time algorithm has been obtained in the last 60 years for general graphs. We break this longstanding barrier by giving an O(n2)-time algorithm on general, integer-weighted graphs. Combined with a popular complexity assumption, we establish a counter-intuitive separation: all-pairs max-flows are strictly easier to compute than all-pairs shortest-paths.Our algorithm produces a cut-equivalent tree, known as the Gomory-Hu tree, from which the max-flow value for any pair can be retrieved in near-constant time. For unweighted graphs, we refine our techniques further to produce a Gomory-Hu tree in the time of a poly-logarithmic number of calls to any maxflow algorithm. This shows an equivalence between the all-pairs and single-pair max-flow problems, and is optimal up to polylogarithmic factors. Using the recently announced m1+o(1)-time max-flow algorithm (Chen et al., March 2022), our Gomory-Hu tree algorithm for unweighted graphs also runs in m1+o(1)-time.
