An in-place, subquadratic algorithm for permutation inversion

TL;DR

This paper introduces a deterministic in-place permutation inversion algorithm with subquadratic time complexity, improving over traditional methods that are quadratic or randomized, thus offering a more efficient solution for large permutations.

Contribution

The paper presents the first deterministic in-place permutation inversion algorithm with a time complexity of O(n^{3/2}), surpassing previous quadratic or randomized approaches.

Findings

Deterministic algorithm runs in O(n^{3/2}) time

In-place inversion uses only O(log n) bits of extra memory

Improves worst-case performance over existing methods

Abstract

We assume the permutation $\pi$ is given by an $n$-element array in which the $i$-th element denotes the value $\pi(i)$. Constructing its inverse in-place (i.e. using $O(\log{n})$ bits of additional memory) can be achieved in linear time with a simple algorithm. Limiting the numbers that can be stored in our array to the range $[1...n]$ still allows a straightforward $O(n^2)$ time solution. The time complexity can be improved using randomization, but this only improves the expected, not the pessimistic running time. We present a deterministic algorithm that runs in $O(n^{3/2})$ time.