本文探讨了三种实现二级排序的方法：按A类别排序后再按B子类别排序。第一种方法是在A排序后，对相同A值的元素按B排序；第二种方法是使用比较函数同时按A和B排序；第三种方法是先稳定排序B再排序A。
how would you implement a secondary sorting. Meaning sorting by Category A, and then sub sorting by category B?
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Three approaches come to mind.
1) Sort on A, then scan the list for runs of elements with the same value for A, then sort each sublist for field B.
2) Sort on A/B simultaneously by using a compare function or key function that uses both fields.
3) Sort the list on B, then stable-sort the list on A.
A nice feature of approach #1 is that you can do it lazily, i.e. only sort the sublists when the second field is queried.
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