探讨了哈希函数的弱点及如何通过随机选择解决冲突问题。介绍了通用哈希的概念，即从键集到有限整数集合的映射，并讨论了完美哈希策略，该策略在二级表格中为一级槽中的项分配足够空间。
Weakness of hashing:
For any choice of hash function, exists bad set of keys that all hash to same slot.
Idea:Choose hash function at random, independently from keys.
Universal hashing:
Define: Let U be a universe of keys, and let H be a finite collection of hash functions mapping U to {0,1,...,m-1}.
Perfect hashing:
If ni items that hash to level-1 slot i, then use mi=ni2 slots in level-2 table.
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