随机森林是什么？
We assume that the user knows about the construction of single classification trees. Random
Forests grows many classification trees. To classify a new object from an input vector, put the
input vector down each of the trees in the forest. Each tree gives a classification, and we say the
tree "votes" for that class. The forest chooses the classification having the most votes (over all the
trees in the forest).
简单来讲，就是根据数据建立多个决策树，当有一个新的观测进入时，带入决策树，
统计决策树不同分类结果所占比例，将观测列入占比例最大的结果中。