What actions will you take to accomplish the "undo"?

Put a substring onto a stack, but make sure to have a flag of whether the text was added or deleted from the overall string.

Substring based on the length of the previous element

Remove text that was inserted from the document using substring

Pop off of the undo stack and onto the redo stack

Remove length of inserted String at stored position.

Pop off one stack and push it onto another.

Substrings and concat