The divide-and-conquer framework, used extensively in classical algorithm design, recursively breaks a problem of size n into smaller subproblems (say, a copies of size n/b each), along with some auxiliary work of cost Caux(n), to give a recurrence relation C(n)≤aC(n/b)+Caux(n)
Nearby in the stack
for the classical complexity
C(n)
. We describe a quantum divide-and-conquer framework that, in certain cases, yields an analogous recurrence relation
CQ(n)≤aCQ(n/b)+O(CQaux(n))
that characterizes the quantum query complexity. We apply this framework to obtain near-optimal quantum query complexities for various string problems, such as (i) recognizing regular languages; (ii) decision versions of String Rotation and String Suffix; and natural parameterized versions of (iii) Longest Increasing Subsequence and (iv) Longest Common Subsequence.