In this paper we consider time and space complexity of languages recognized by two-way deterministic pushdown automata with k input heads (2dpda(k)‘s, for short). Using the pyramidal structure of Schonhage [7] we design an algorithm simulating 2dpda(k)‘s in O(r) time and space, where r is the number of reachable (during the computation for a given input word) surface configurations. If the computation of 2dpda(k) is ‘sparse’, then r is an improvement upon O(nk). This generalizes the result of… CONTINUE READING