Description of Change

Added Preorder and Postorder Traversal functions in Binary Search Tree. In previous version there was only Inorder Traversal. I thought that these operations are also important since they used in various binary search problems like cloning bst (preorder) and deleting bst (postorder)

References

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
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• I acknowledge that all my contributions will be made under the project's license.

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