Multivariate generalizations of the Wald-Wolfowitz runs statistic and the Smirnov maximum deviation statistic for the two-sample problem are presented. They are based on the minimal spanning tree of the pooled sample points. Some null distribution results are derived and a simulation study of power is reported.
"Multivariate Generalizations of the Wald-Wolfowitz and Smirnov Two-Sample Tests." Ann. Statist. 7 (4) 697 - 717, July, 1979. https://doi.org/10.1214/aos/1176344722