An in-place, subquadratic algorithm for permutation inversion

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 $[\mathrm{1...}n]$ 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.