Ralf Hinze and Ross Paterson. in Journal of Functional Programming16:2 (2006), pages 197-217.
We present 2-3 finger trees, a functional representation of persistent sequences supporting access to the ends in amortized constant time, and concatenation and splitting in time logarithmic in the size of the smaller piece. Representations achieving these bounds have appeared previously, but 2-3 finger trees are much simpler, as are the operations on them. Further, by defining the split operation in a general form, we obtain a general purpose data structure that can serve as a sequence, priority queue, search tree, priority search queue and more.
General techniques for designing functional data structures, with a wealth of applications. Finger trees can be viewed as a generalization of the implicit deques discussed in section 11.1.
A more complex structure achieving the same bounds, but in the worst case. They also sketch a variant that is claimed to achieve doubly logarithmic time for concatenation.