I read "Data Structures & Algorithms I Actually Used Working at Tech Companies". After reading, I just pick one thing that i want to know with details. So i choose "Hash" because it is used widely and frequently.
A hash function is any function that can be used to map data of arbitrary size to fixed-size values. The values are used to index a fixed-size table called a hash table. [1]
Hashing could have collision where different values can be same result. We can use space more or re-calculate for fix this collision.