Improve examples

[czgdp1807]

Description of the problem

In the following snippet,

pydatastructs/pydatastructs/linear_data_structures/linked_lists.py

Lines 39 to 53 in 18524da

	>>> from pydatastructs import DoublyLinkedList
	>>> dll = DoublyLinkedList()
	>>> dll.append(6)
	>>> dll[0].data
	6
	>>> dll.head.data
	6
	>>> dll.append(5)
	>>> dll.append_left(2)
	>>> print(dll)
	[2, 6, 5]
	>>> dll[0].data = 7.2
	>>> dll.extract(1).data
	6
	>>> print(dll)

print has been used for showing string output, instead, str should have been used. Changes should be made in the above part of the code as well as such things should be checked in other parts of the codebase as well and a single PR should be made for such improvements.

Example of the problem

References/Other comments

[czgdp1807]

Some classes don't have examples. This can be done in a separate PR other than one doing the above fixes.

[iamrajiv]

@czgdp1807 changing print() with str() is giving DocTestFailure. For example str() is returning like this '[7.2, 5]'

[czgdp1807]

Yeah so you can make changes to the results by adding '.