# HASHSETS

A set is a collection of distinct elements, and it is a fundamental concept in mathematics. It is often used to describe relationships between objects, and it is an important tool for mathematical reasoning and problem-solving.

In programming, a set can be implemented as a data structure that allows the efficient storage, retrieval, and manipulation of elements. The most common operations for sets include adding and removing elements, checking for membership, and computing the union, intersection, and difference of sets.

Sets can be finite or infinite, and they can be described using various notations, such as listing the elements, using set-builder notation, or using Venn diagrams to illustrate the relationships between sets.

In set theory, there are several important concepts, including subsets, proper subsets, the empty set, the universal set, and the power set. Sets can be combined using operations like union, intersection, and difference, and they can be compared using the concepts of equality and inclusion.

Set theory is used in various branches of mathematics, including algebra, analysis, and topology, and it is also applied in computer science, statistics, and other fields.

There are different algorithms for performing set operations such as union, intersection, difference, and complement. Some of these algorithms are based on set theory, while others are specific to the data structure being used.

Sets have different applications in computer science such as data compression, searching, sorting, and database systems. Understanding sets and their properties is important for developing efficient algorithms and data structures.

In computer science, a hash set is a data structure that uses a hash table to store a set of values. A hash set is similar to a hash map, but it only stores keys, without any associated values.

A hash set works by taking each element that is added to it, hashing the element to determine a unique index in the hash table, and then storing the element in the corresponding location in the table. When we want to check if an element is in the set, we hash the element to determine the index in the table, and then check if the element is stored at that location.

Hash sets are efficient for adding and checking the existence of an element in constant time on average. However, iterating through the elements in a hash set can be less efficient than iterating through elements in other data structures, since the elements are not stored in a particular order.

## PATTERNS

### COMPLEMENT APPROACH

The complement approach pattern is a technique used to solve problems by transforming them into problems involving complements. The complement of a set A is the set of elements that are not in A, but are in the universe that A is a subset of.

In computer science, this pattern is often used in problems that require finding pairs of elements that complement each other in some way. For example, finding two numbers in an array that add up to a given target value.

The complement approach works by transforming the problem into a complement problem that is easier to solve. For example, instead of finding two numbers that add up to a given target value, we can find two numbers that add up to the complement of the target value (i.e., the difference between the target value and the sum of the two numbers).

To solve the complement problem, we can use a hash set to store the values in the input set, and then iterate over the set again to check for complements. If we find a complement, we can return the corresponding pair of elements.

The complement approach pattern can also be used to solve other types of problems, such as finding all elements in an array that do not satisfy a certain condition, or finding all subsets of a set that do not contain a certain element. In each case, the problem is transformed into a complement problem that is easier to solve using a hash set or other data structure.

### BACKTRACKING

Backtracking can be also applied to problems regarding sets a common example is the problem of finding all the subsets of a given array of numbers:

```
```public List<List<Integer>> subsets(int[] nums) {

List<List<Integer>> result = new
ArrayList<>();

backtrack(result, new ArrayList<>(), nums,
0);

return result;

}

private void backtrack(List<List<Integer>> result, List<Integer>
temp, int[] nums, int start) {

result.add(new ArrayList<>(temp));

for (int i = flex-start; i < nums.length; i++) {

temp.add(nums[i]);

backtrack(result, temp, nums, i +
1);

temp.remove(temp.size() - 1);

}

}

In this implementation, the backtrack function recursively generates all possible subsets by adding each element of the input set to a temporary list, and then calling itself recursively with the next element in the input set. The base case is reached when all elements have been added to the temporary list, and the function adds the current list to the result list. The temporary list is then backtracked by removing the last element, so that the function can try another combination of elements. The algorithm runs in O(2^N) time complexity, where N is the size of the input set, because it generates all possible subsets of the set.

### UNION FIND

Union-find is a data structure that provides an efficient way to perform two operations on a set of elements:

- Union: merge two sets into one set.
- Find: determine which set an element belongs to.

The union-find data structure is often used to keep track of a partition of a set. The elements in the set can be represented by integers, and each integer is associated with a node in a tree. The root of the tree is the representative element of the set.

Initially, each element is in a set by itself, and the tree for each element has only one node. When two sets are merged, the root of one tree is made the child of the root of the other tree, and the two trees become one tree.

The union-find data structure provides an efficient way to perform both the union and find operations. The find operation finds the root of the tree that contains the element, which is also the representative element of the set. The union operation merges two trees by making the root of one tree the child of the root of the other tree.

The efficiency of the union-find data structure is important, as it is often used in algorithms that operate on large sets of data. One common use of the union-find data structure is in Kruskal's algorithm for finding the minimum spanning tree of a graph.

## PROBLEMS

Easy

Medium

- Intersection of Two Arrays
- Valid Sudoku
- Longest Consecutive Sequence
- Intersection of Two Arrays II
- Longest Substring Without Repeating Characters
- Group Anagrams

Hard