Added README.md for union find

Data Structures and Algorithms/union_find/README.md

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+# Union Find (Disjoint Set Union) - Implementation and Use
+
+## Table of Contents
+- [Why Union Find?](#why-union-find)
+- [Functions and Examples](#functions-and-examples)
+- [Setup](#setup)
+- [Additional Resources](#additional-resources)
+- [Leetcode Questions](#leetcode-questions)
+
+## Why Union Find?
+Union Find is a popular data structure that allows us to solve many different types of graph
+problems. It works best with undirected graphs, and it allows us to figure out whether a node
+is connected to another node.
+
+Some problems it can be used to solve:
+- Find the minimum spanning tree in a graph (Kruskal's)
+- Check if there is a path between two nodes
+- Finding redundant edges 
+- Representing networks 
+
+
+## Functions and Examples
+Union Find seems complex at first, but it is actually a lot easier when you understand that there are 
+only two functions.
+- Find(n) : returns the parent of a node n
+- Union(n1, n2) : connects n1 and n2 if they are not previously connected
+
+Let's look at an example!  
+```python
+u = UnionFind(7)  # create a UnionFind object with 7 nodes (numbered 0 to 6)
+
+u.union(0, 1)     # connects 0 and 1 together
+u.union(5, 6)     # connects 5 and 6 together
+
+u.find(1)         # returns 0, since 0 is parent of 1
+u.find(5)         # returns 5, since 5 is its own parent
+
+u.union(1, 2)     # connects 2 to the component 0-1
+u.find(2)         # 2s parent is now 0
+
+# Now our structure looks like this
+
+# 0-1-2   3  4  5-6
+
+u.union(1, 6)    # first we find the parents of 1 and 6
+                 # parents are 0, and 5
+                 # connect the smaller component to the bigger
+                 # now 5's parent is 0
+
+u.find(6)       # now this goes:
+                # 6 parent is 5 -> 5 parent is 0 -> 0 is its own parent
+```
+
+And that's it! You can use the sample code to test different examples with Union Find.
+In the code, par keeps track of the parent of each node and rank keeps track of the size of 
+each component.
+
+## Setup
+
+First clone the repo
+ > `cd union_find` to get into this folder.   
+ >  call the verify function anywhere, consider adding ``` if __name__ == '__main__'```  
+ > `python union_find.py` to run the demo
+
+ You can modify the structure in the verify function and play around with it.
+
+ ## Additional Resources
+
+ Here are some resources I found useful when learning: 
+ - Neetcode Graph Videos on YouTube 
+ - William Fiset - Union Find Video on YouTube
+ - Union Find Medium Article by Claire Lee 
+ - Union Find Visualizer - Visualgo 
+
+ ## Leetcode Questions
+ - 200 - Number of Islands
+ - 684 - Redundant Connection
+ - 695 - Max Area of an Island
+ - 827 - Making a Large Island 
+ - 2316 - Count Unreachable Pairs of Nodes in an Undirected Graph
+ - 2421 - Maximum Score of a Good Path
+ - 2709 - Greatest Common Divisor Traversal 
+
+ I hope this was helpful. If there are any mistakes or issues or if you want to contribute to union find, feel free to contact me at rawateshaan0 [at] gmail [dot] com