Optimize the tree traversal by utilizing memoization

[nizarmah]

• Snapshot directories by traversing root nodes recursively #4

This change can be optimized in the future.

Imagine passing two directories as parameters, one is ~/A and the other is ~/A/B.

At the moment, the program traverse both ~/A and ~/A/B. However, we're traversing ~/A/B twice.

This can be optimized by using memoization. That way we wouldn't have to re-traverse ~/A/B. Instead, we can just point towards that Node.