CW-complexes in the Category of Small Categories

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We compute the collection of CW-complexes in the model category of small categories constructed by Joyal and Tierney. More generally, if X is a connected topological space, we show that the homotopy category of CW-complexes in Joyal-Tierney's model category of sheaves of sets on X is equivalent to the homotopy category of groupoids. As an application of the ideas, we show that the algebraic K-theory groups of the category of pointed small categories are trivial, and more generally, the algebraic K-theory groups of any sufficiently "nice" Waldhausen category A of pointed small categories also vanishes, regardless of finiteness conditions assumed on the objects of A. The vanishing of this K-theory implies that there is no nontrivial Euler characteristic defined on pointed small categories and satisfying certain niceness axioms.