I have used Stack for the iterative Preorder Traversal.

Describe your change:

Steps To Follow-

1. Create an empty stack nodeStack and push root node to stack.
2. Do following while nodeStack is not empty.
   a) Pop an item from stack and print it.
   b) Push right child of popped item to stack
   c) Push left child of popped item to stack

Right child is pushed before left child to make sure that left subtree is processed first.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.