Sets are abstract data structures which are able to store and keep track of unique values.

Imagine you have a grocery list that you use to keep tracking of things you need to buy. You want to make sure there are no duplicate items in the list, you can add items to the list and that you can remove items from your list. This structure is similar to what a set does.

Sets have three operations: insertion, deletion and a membership test. Insertion places an element into the set, deletion removes an element from the set and a membership test is checking whether an element exists within the set.


Type Membership Insertion Deletion
Hash Set O(1) O(1) O(1)
Tree Set O(log n) O(log n) O(log n)


  1. Given a list of words, determine how many of them are anagrams of each other. An anagram is a word that can have its letters scrambled into another word.
    • For example, silent and listen are anagrams but banana and orange are not.
  2. Given the friend lists of two people, find the number of mutual friends.
  3. Given an array of numbers, find the number of pairs of numbers that sum to 0.
  4. Given an array of numbers, find the number of tuples of size 4 that add to A.
    • For example in the list (10,5,-1, 3, 4, -6) the tuple of size 4 (-1,3,4-6) adds to 0.