Backward and Bi-Directional Search

• Search so far has started at initial and generated successors.

• Suppose instead we stared at goal and generated predecessors.

  • or suppose we started both at initial and at goal, and went in both directions

• The argument: fan-out versus fan-in

• Difficulties

  • computing predecessors
  • bookeeping to determine when a bi-directional search succeeds

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