*Richard E. Korf*

Best-first search is a general search algorithm that, always expands next a frontier node of lowest cost. Its applicability, however, is limited by its exponential memory requirement. Iterative deepening, a previous approach to this problem, does not expand nodes in best-first order if the cost function can decrease along a path. We present a linear-space best-first search algorithm (RBFS) that always explores new nodes in best-first order, regardless of the cost function, and expands fewer nodes than iterative deepening with a nondecreasing cost function. On the sliding-tile puzzles, RBFS with a weighted evaluation function dramatically reduces computation time with only a small penalty in solution cost. In general, RBFS reduces the space complexity of best-first search from exponential to linear, at the cost of only constant factor in time complexity in our experiments.

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