425. Cancel Case Pairs

A text editor has an odd autocorrect rule: whenever two neighbouring letters are the same letter but written in different cases (such as g beside G, in either order), both are deleted at once. Deleting a pair can bring two new letters together, so the rule keeps firing until no such neighbours remain.

Given the string s of English letters, return what is left once nothing more can be cancelled. The result may be an empty string. The order in which pairs are cancelled does not change the outcome, so the answer is unique.