3.1、什么是无监督学习
data only comes with inputs x but not output labels y. algorithm has to find structure in the data.
3.2、类型
3.2.1、聚类 clustering
group similar data points together.
3.2.2、降维 dimensionality reduction
compress data using fewer numbers.
3.2.3、欺诈检测 anomaly detection
find unusual data points.