上QQ阅读APP看书，第一时间看更新
One-vs-one
The alternative to one-vs-all is training a model for each pair of classes. The complexity is no longer linear (it's O(n2) indeed) and the right class is determined by a majority vote. In general, this choice is more expensive and should be adopted only when a full dataset comparison is not preferable.
If you want to learn more about multiclass strategies implemented by scikit-learn, visit
http://scikit-learn.org/stable/modules/multiclass.html.
http://scikit-learn.org/stable/modules/multiclass.html.