For isn't "linear traversal", it's linear execution. It's up to the data structure to define what linear traversal is. For arrays and lists that's trivial. For

For isn't "linear traversal", it's linear execution. It's up to the data structure to define what linear traversal is. For arrays and lists that's trivial. For trees and, more generally, graphs there are many ways to do traversal. This is what iterators are for.