unordered_set VS set

在stackoverflow上看到一个问题，大概是问为什么有了平均查询O（1）的unordered_set我们还需要平均O（logN）的set呢？

首先我们需要知道set在STL底层是用红黑树来实现的，而unordered_set是基于hash。

在这里记录一下比较好的答案。

• set uses less memory than unordered_set to store the same number of elements.（set更节省空间）

• For a small number of elements, lookups in a set might be faster than lookups in an unordered_set.（小规模的话set或许更快）Even though many operations are faster in the average case for unordered_set, they are often guaranteed to have better worst case complexities for set (for example insert).（但总的来说基于hash的unordered_set还是快些）

• That set sorts the elements is useful if you want to access them in order.
  You can lexicographically compare different sets with <, <=, > and >=. unordered_sets are not required to support these operations.（set有序输出）

• People tend to overlook the fact that hashtables have O(1) average-case access time, meaning they can occasionally have big delays. The distinction can be important for real-time systems.
  （这一点也很重要，对于实时系统来说tree的最糟糕的情况还是比hash好）