编写代码计算信息增益，splitDataSet函数是用来选择各个特征的子集的，比如选择年龄(第0个特征)的青年(用0代表)的自己，我们可以调用splitDataSet(dataSet,0,0)这样返回的子集就是年龄为青年的5个数据集。chooseBestFeatureToSplit是选择选择最优特征的函数。(Write code to calculate the information gain.SplitDataSet function is used to select the feature subset, such as the choice of age (0) young (0), we can call splitDataSet (dataSet, 0, 0) returns a subset of the five data sets is the age of youth. ChooseBestFeatureToSplit is a function that selects the best features.)