SIAM Journal on Computing (to appear) 1996

Linear and O(n log n) Time Minimum-Cost Matching Algorithms for Quasi-convex Tours

Samuel R. Buss and Peter N. Yianilos

Abstract: Let G be a complete, weighted, undirected, bipartite graph with n red nodes, n' blue nodes, and symmetric cost function c(x,y). A maximum matching for G consists of min(n,n') edges from distinct red nodes to distinct blue nodes. Our objective is to find a minimum-cost maximum matching, i.e., one for which the sum of the edge costs has minimal value. This is the weighted bipartite matching problem; or as it is sometimes called, the assignment problem.

We report a new and very fast algorithm for an abstract special case of this problem. Our first requirement is that the nodes of the graph are given as a quasi-convex tour. This means that they are provided circularly ordered as x_1,...,x_N where N = n + n', and that for any x_i, x_j, x_k, x_l, not necessarily adjacent but in tour order, with x_i,x_j of one color and x_k,x_l of the opposite color, the following inequality holds:

c(x_i,x_l) + c(x_j,x_k) <= c(x_i,x_k) + c(x_j,x_l)

If n = n', our algorithm then finds a minimum-cost matching in O(N log N) time. Given an additional condition of weak analyticity, the time complexity is reduced to O(N). In both cases only linear space is required. In the special case where the circular ordering is a line-like ordering, these results apply even if n != n'.

Our algorithm is conceptually elegant, straightforward to implement, and free of large hidden constants. As such we expect that it may be of practical value in several problem areas.

Many natural graphs satisfy the quasi-convexity condition. These include graphs which lie on a line or circle with the canonical tour ordering, and costs given by any concave-down function of arclength --- or graphs whose nodes lie on an arbitrary convex planar figure with costs provided by Euclidean distance.

The weak-analyticity condition applies to points lying on a circle with costs given by Euclidean distance, and we thus obtain the first linear-time algorithm for the minimum-cost matching problem in this setting (and also where costs are given by the L_1 or L_\infty metrics).

Given two symbol strings over the same alphabet, we may imagine one to be red and the other blue, and use our algorithms to compute string distances. In this formulation, the strings are embedded in the real line and multiple independent assignment problems are solved; one for each distinct alphabet symbol.

While these examples are somewhat geometrical, it is important to remember that our conditions are purely abstract; hence, our algorithms may find application to problems in which no direct connection to geometry is evident.

Keywords: assignment problem, bipartite weighted matching, computational geometry, concave penalty function, convexity, linear time, Monge property, quadrangle inequality, string comparison.


A preliminary version of this paper appeared in the Proceedings of the Fifth Annual ACM-SIAM Symposium on Discrete Algorithms, January 1994, pp.65-76.

The manuscript was completed in May of 1993 and was released as an NEC Research Institute Technical Report. report.