两个点集之间距离的方法有三种度量方式：
Single Linkage
The distance between two clusters is defined as the shortest distance between two points in each cluster. For example, the distance between clusters “r” and “s” to the left is equal to the length of the arrow between their two closest points.
Complete Linkage
The distance between two clusters is defined as the longest distance between two points in each cluster. For example, the distance between clusters “r” and “s” to the left is equal to the length of the arrow between their two furthest points.
Average Linkage
The distance between two clusters is defined as the average distance between each point in one cluster to every point in the other cluster. For example, the distance between clusters “r” and “s” to the left is equal to the average length each arrow between connecting the points of one cluster to the other.
参考文献:
[1] Hierarchical Clustering
[2] 层次聚类算法的原理及实现Hierarchical Clustering
[3] 官方文档
[4] 机器学习—聚类系列-层次聚类（Hierarchical Clustering）