Add README.

Author: trekhleb

src/data-structures/graph/README.md

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 # Graph
 
+In computer science, a graph is an abstract data type 
+that is meant to implement the undirected graph and 
+directed graph concepts from mathematics, specifically
+the field of graph theory
+
 A graph data structure consists of a finite (and possibly 
 mutable) set of vertices or nodes or points, together 
 with a set of unordered pairs of these vertices for an 

src/data-structures/hash-table/README.md

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+# Hash Table
+
+In computing, a hash table (hash map) is a data 
+structure which implements an associative array 
+abstract data type, a structure that can map keys 
+to values. A hash table uses a hash function to 
+compute an index into an array of buckets or slots, 
+from which the desired value can be found
+
+Ideally, the hash function will assign each key to a 
+unique bucket, but most hash table designs employ an 
+imperfect hash function, which might cause hash 
+collisions where the hash function generates the same
+index for more than one key. Such collisions must be
+accommodated in some way.
+
+![Hash Table](https://upload.wikimedia.org/wikipedia/commons/7/7d/Hash_table_3_1_1_0_1_0_0_SP.svg)
+
+Hash collision resolved by separate chaining.
+
+![Hash Collision](https://upload.wikimedia.org/wikipedia/commons/d/d0/Hash_table_5_0_1_1_1_1_1_LL.svg)
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Hash_table)