Added doctest & docstring to quadratic_probing.py (#10996)

* Added doctest & docstring to quadratic_probing.py

* Update quadratic_probing.py

* Update quadratic_probing.py

data_structures/hashing/quadratic_probing.py

@@ -12,6 +12,55 @@ class QuadraticProbing(HashTable):
         super().__init__(*args, **kwargs)
 
     def _collision_resolution(self, key, data=None):
+        """
+        Quadratic probing is an open addressing scheme used for resolving
+        collisions in hash table.
+
+        It works by taking the original hash index and adding successive
+        values of an arbitrary quadratic polynomial until open slot is found.
+
+        Hash + 1², Hash + 2², Hash + 3² .... Hash + n²
+
+        reference:
+            - https://en.wikipedia.org/wiki/Quadratic_probing
+        e.g:
+        1. Create hash table with size 7
+        >>> qp = QuadraticProbing(7)
+        >>> qp.insert_data(90)
+        >>> qp.insert_data(340)
+        >>> qp.insert_data(24)
+        >>> qp.insert_data(45)
+        >>> qp.insert_data(99)
+        >>> qp.insert_data(73)
+        >>> qp.insert_data(7)
+        >>> qp.keys()
+        {11: 45, 14: 99, 7: 24, 0: 340, 5: 73, 6: 90, 8: 7}
+
+        2. Create hash table with size 8
+        >>> qp = QuadraticProbing(8)
+        >>> qp.insert_data(0)
+        >>> qp.insert_data(999)
+        >>> qp.insert_data(111)
+        >>> qp.keys()
+        {0: 0, 7: 999, 3: 111}
+
+        3. Try to add three data elements when the size is two
+        >>> qp =  QuadraticProbing(2)
+        >>> qp.insert_data(0)
+        >>> qp.insert_data(999)
+        >>> qp.insert_data(111)
+        >>> qp.keys()
+        {0: 0, 4: 999, 1: 111}
+
+        4. Try to add three data elements when the size is one
+        >>> qp =  QuadraticProbing(1)
+        >>> qp.insert_data(0)
+        >>> qp.insert_data(999)
+        >>> qp.insert_data(111)
+        >>> qp.keys()
+        {4: 999, 1: 111}
+        """
+
         i = 1
         new_key = self.hash_function(key + i * i)
 
@@ -27,3 +76,9 @@ class QuadraticProbing(HashTable):
                 break
 
         return new_key
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()