Solving problems by searching

Chapter 3 addendum

(Bidirectional search)

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(b^d/2) is smaller than O(b^d)

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.


	Simultaneously search both forward (from the initial state) and backward (from the goal state)
	Stop when the two searches meet.
	Intuition = 2 * O(b^d/2) is smaller than O(b^d)


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.