solutions: 0606 - Construct String from Binary Tree (Easy)[Java]

[kesava-karri]

Change Summary

• Added Java solution

Checklist

If you haven't fulfilled the below requirements or even delete the entire checklist, your PR won't be reviewed and will be closed without notice. Regular contributors (with 10+ PRs) can skip this part.

General

• [x] This Pull Request is all my own work. (You'll be blacklisted if you are caught for plagiarism.)
• [x] I've read CONTRIBUTING.md
• [ ] I've applied LaTex for all variables, formulas and time / space complexity instead of using backticks
• [x] I've started the app locally and verified all the content and all links (if applicable) are accessible correctly
• [ ] I've included Complexity Analysis (Time Complexity & Space Complexity).
• [x] I've written my explanation well and it is easy to understand for beginners

Tutorial

• [ ] I've read and followed the Tutorial Template
• [ ] I've explained my topic well with 2 - 3 LC problems and no external problems are used.
• [ ] I've provided the full working solutions to the problems used in this tutorial.
• [ ] I've provided suggested problems at the end with the given format. See here as an example. If the target solution is not available, leave solutionLink blank.
• [ ] I've given credits / references if I use external resources. (For an image, give credit under it. Otherwise, add a new section called References at the end (after Suggested Problems).)

Solutions

• [x] I've read and followed the Solution Template
• [x] I've formatted my code well with K&R Coding style
• [x] I've confirmed that comments are put above each line rather than writing on the same line.
• [x] I've included a meaningful approach name for my solution. e.g. ## Approach 1: Two Pointers.

[kesava-karri]

Thanks for the swift review