Recognizing well-parenthesized expressions in the streaming model

TL;DR

This paper develops optimal streaming algorithms for recognizing well-parenthesized expressions with multiple types of parentheses, demonstrating significant space savings with multi-pass and reverse input access, and establishing tight lower bounds.

Contribution

It introduces the first near-optimal one-pass randomized streaming algorithm for Dyck(2), along with a highly space-efficient two-pass algorithm utilizing reverse input access, and proves matching lower bounds.

Findings

One-pass algorithm uses O(√n log n) space, optimal up to polylog factors.

Two-pass algorithm reduces space to O((log n)^2) with reverse input.

Lower bounds confirm the optimality of the algorithms within polylog factors.

Abstract

Motivated by a concrete problem and with the goal of understanding the sense in which the complexity of streaming algorithms is related to the complexity of formal languages, we investigate the problem Dyck(s) of checking matching parentheses, with $s$ different types of parenthesis. We present a one-pass randomized streaming algorithm for Dyck(2) with space $\Order(\sqrt{n}\log n)$, time per letter $\polylog (n)$, and one-sided error. We prove that this one-pass algorithm is optimal, up to a $\polylog n$ factor, even when two-sided error is allowed. For the lower bound, we prove a direct sum result on hard instances by following the "information cost" approach, but with a few twists. Indeed, we play a subtle game between public and private coins. This mixture between public and private coins results from a balancing act between the direct sum result and a combinatorial lower bound for the base case. Surprisingly, the space requirement shrinks drastically if we have access to the input stream in reverse. We present a two-pass randomized streaming algorithm for Dyck(2) with space $\Order((\log n)^2)$, time $\polylog (n)$ and one-sided error, where the second pass is in the reverse direction. Both algorithms can be extended to Dyck(s) since this problem is reducible to Dyck(2) for a suitable notion of reduction in the streaming model.