Judy is a C library that provides a state-of-the-art core technology
that implements a sparse dynamic array.
是否真的如作者说的那么快？
A Judy tree is generally faster than and uses less memory than
contemporary forms of trees such as binary (AVL) trees, b-trees,
and skip-lists. When used in the "Judy Scalable Hashing" configuration,
Judy is generally faster then a hashing method at all populations.
反正10分钟我是看不懂作者说的原理的：
先留着，以后有用。