Just because you don't have a keyboard doesn't mean you can't have access to the iPad's "undo" or "redo" functions. (Normally Command+Z and Command+Shift+Z on an iPad-compatible keyboard)
For starters, you can still do the old iPhone trick and shake your iPad for a quick undo. But if you're worried about sending your precious iPhone flying straight at an innocent bystander's noggin, then the touchscreen keyboard works, too.
First, bring up the touchscreen keyboard and hit the ".?123" button. This brings up another set of virtual keyboard buttons, including an "undo" button that you can click to your undoing heart's content.
For a redo, press "#+=" and you'll bring up the "redo" button.
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