Notes
Slide Show
Outline
1
Solving problems by searching
  • Chapter 3 addendum
  • (Bidirectional search)
2
Bidirectional Search
  • Simultaneously search both forward (from the initial state) and backward (from the goal state)
  • Stop when the two searches meet.
  • Intuition = 2 * O(bd/2) is smaller than O(bd)
3
Bidirectional Search Discussion
  • Numerical Example (b=10, l = 5)
    • Bi-directional search finds solution at d=3 for both forward and backward search.  Assuming BFS in each half 2222 nodes are expanded.
  • Implementation issues:
    • Operators are reversible, e.g., Pred(Succ(n)) = Pred(Succ(n))
    • There may be many possible goal states.
      • Construct a goal state containing the superset of all goal states.
    • Check if a node appears in the “other” search tree.
    • Using different search strategies for each half.