Wavelet Trees Meet Suffix Trees

We present an improved wavelet tree construction algorithm and discuss its applications to a number of rank/select problems for integer keys and strings. Given a string of length n over an alphabet of size σ ≤ n, our method builds the wavelet tree in O(n log σ/ √ logn) time, improving upon the state-of-the-art algorithm by a factor of √ log n. As a consequence, given an array of n integers we can construct in O(n √ logn) time a data structure consisting of O(n) machine words and capable of answering rank/select queries for the subranges of the array in O(log n/ log log n) time. This is a log logn-factor improvement in query time compared to Chan and Pătraşcu (SODA 2010) and a √ logn-factor improvement in construction time compared to Brodal et al. (Theor. Comput. Sci. 2011). Next, we switch to stringological context and propose a novel notion of wavelet suffix trees. For a string w of length n, this data structure occupies O(n) words, takes O(n√logn) time to construct, and simultaneously captures the combinatorial structure of substrings of w while enabling efficient top-down traversal and binary search. In particular, with a wavelet suffix tree we are able to answer in O(log |x|) time the following two natural analogues of rank/select queries for suffixes of substrings: 1) For substrings x and y of w (given by their endpoints) count the number of suffixes of x that are lexicographically smaller than y; 2) For a substring x of w (given by its endpoints) and an integer k, find the k-th lexicographically smallest suffix of x. We further show that wavelet suffix trees allow to compute a run-length-encoded BurrowsWheeler transform of a substring x of w (again, given by its endpoints) in O(s log |x|) time, where s denotes the length of the resulting run-length encoding. This answers a question by Cormode and Muthukrishnan (SODA 2005), who considered an analogous problem for LempelZiv compression. All our algorithms, except for the construction of wavelet suffix trees, which additionally requires O(n) time in expectation, are deterministic and operate in the word RAM model. ∗The third author is supported by Polish budget funds for science in 2013-2017 as a research project under the ‘Diamond Grant’ program. The fourth author is partly supported by Dynasty Foundation.