  We introduce and analyze two parameter-free linear-memory tree search
algorithms. Under mild assumptions we prove our algorithms are guaranteed to
perform only a logarithmic factor more node expansions than A* when the search
space is a tree. Previously, the best guarantee for a linear-memory algorithm
under similar assumptions was achieved by IDA*, which in the worst case expands
quadratically more nodes than in its last iteration. Empirical results support
the theory and demonstrate the practicality and robustness of our algorithms.
Furthermore, they are fast and easy to implement.
