Added C++ code for counting leaf nodes on a binary tree

[swetakb15]

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Closes: #[issue number that will be closed through this PR]

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Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Code style update (formatting, local variables)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

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• [ ] My code follows the style guidelines of this project.
• [ ] I have performed a self-review of my own code.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [ ] I have made corresponding changes to the documentation.
• [ ] My changes generate no new warnings.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] New and existing unit tests pass locally with my changes.

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[ankitapuri]

please create the folder for trees and separate folder for cpp in it and add it there.