Replace tests of binary tree with in-order and pre-order

[czgdp1807]

Description of the problem

As of now, the correctness of operations on a binary tree is determined by verifying the str(tree) with the expected string. This makes it quite difficult to check the tests. Instead of using, str(tree), in-order and pre-order should be checked since, using both these traversals a binary tree can be uniquely constructed.

Example of the problem

See the tests of RedBlackTree.

References/Other comments

[1] #293

[Vanshikagarg17]

I am from GSSoC 2020, I would like to work upon this.

[prshnt19]

@Vanshikagarg17 Sure. Just have a look at https://github.com/codezonediitj/pydatastructs#how-to-contribute if not already done so.

[rsriramc]

Can I take up on this for KWoc 2020, or is it closed (since there was a pull request) ?

[czgdp1807]

Sure.

[rsriramc]

I have opened this Pull request #304 which solves this issue.