Abstrac tThe Eight Queens Problem is used to illustrate some different approaches to recursiv e programming and parallel processing .The Eight Queens Problem is a familiar example in programming and algorithm textbooks . Usually, the given solution applies depth-first search with backtracking, and stops afte r finding one solution . When we attempted to reformulate the problem by applyin g breadth-first search, in order to find all solutions without backtracking, we were only abl e to find one published example, written in a Miranda-like functional language[1] .In the present article, we show the Lisp solution for the problem, and discuss som e programming issues arising from the effort . Usually, the backtracking program is used a s an illustration of recursion : A tree is being searched, and if one branch leads to a n unsatisfactory queen placement, then we move back to a higher node and try anothe r branch. The breadth-first algorithm is iterative rather than recursive : Nodes at the sam e depth are examined one by one, and those found to be satisfactory are extended to th e next level, while unsatisfactory ones are removed . The same node by node search is then performed at the next level . It is also clear that a suitable data structure, with dynami c storage management to extend wanted nodes and remove unwanted ones, must b e available to support the iteration, whereas in recursion dynamic storage is achieved through the varying depth of the stack with procedure entries and exits . However, in ou r Lisp program we will use recursion to perform iteration .
