A*+BFHS: A Hybrid Heuristic Search Algorithm
  We present a new algorithm A*+BFHS for solving hard problems where A* and
IDA* fail due to memory limitations and/or the existence of many short cycles.
A*+BFHS is based on A* and breadth-first heuristic search (BFHS). A*+BFHS
combines advantages from both algorithms, namely A*'s node ordering, BFHS's
memory savings, and both algorithms' duplicate detection. On easy problems,
A*+BFHS behaves the same as A*. On hard problems, it is slower than A* but
saves a large amount of memory. Compared to BFIDA*, A*+BFHS reduces the search
time and/or memory requirement by several times on a variety of planning
domains.
