Sorting Multisets Stably in Minimum Space

In a decision tree model, Ω(n log2n − ∑ m i=1 ni log2ni + n) is known to be a lower bound for sorting a multiset of size n containing m distinct elements, where the ith distinct element appears n t times. We present a minimum space algorithm that sorts stably a multiset in asymptotically optimal worst-case time. A Quicksort type approach is used, where at each recursive step the median is chosen as the partitioning element. To obtain a stable minimum space implemention, we develop linear-time in-place algorithms for the following problems, which have interest of their own: Stable unpartitioning: Assume that an n-element array A is stably partitioned into two subarrays A0 and A1. The problem is to recover A from its constituents A0 and A1. The information available is the partitioning element used and a bit array of size n indicating whether an element of A0 or A1 was originally in the corresponding position of A.