默认度是 1，这就是为什么在继承树中一些子类会丢失的原因。
The default degree is 1, and this is why some grandchildren are missed in the inheritance tree.
树是设计中最具象征意义的图案之一，原因可能在于它的普及度相当高。
Trees are one of the most symbolic pieces of design and probably the reason for their popularity.
决策树解决方法的复杂度是多少？
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