How to call a recursive method?

Question

Java

Good day.
With the help of recursion, I need to find the reverse in order of direct and centered traversal of a binary tree.
For example,

preordered = "ABCDGEFHIK"
inordered = "DGCBFEAKIH"

My problem is that when calling methods for processing the left and right subtrees relative to root A, I need to pass only DGCBFE and KIH to the methods, respectively. How can you pass to such a method when it is called a part of the array / list of characters, starting / ending with a certain index?