Bidirectional search n two simultaneous searches from

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Unformatted text preview: eached the given depth without finding a solution Evaluation of IDS n༆  Evaluation of IDS Completeness: YES n༆  Time complexity: O(bd) n༆  Completeness: q༇  YES Time complexity: O(bd) n༆  Space complexity: O(bd) q༇  n༆  q༇  Same as DFS 11 9/20/13 Evaluation of IDS n༆  Completeness: YES Time complexity: O(bd) Space complexity: O(bd) Iterative Deepening Search n༆  q༇  n༆  n༆  q༇  n༆  Same as DFS Optimality: q༇  YES if step cost is 1. Bidirectional Search n༆  Two simultaneous searches from start and goal. Motivation: bd/2 + bd/2 much less than bd Before a node is expanded it is checked if it is in the fringe of the other search (can be done in constant time using a hash table). Need to keep at least one of the search trees in memory Time complexity: O(bd/2). Space complexity: same. Complete and optimal (for uniform step costs) if both searches are BFS q༇  n༆  n༆  n༆  n༆  n༆  n༆  Analogous to BFS: explores a complete layer of nodes before proceeding to the next one. Combines benefits of DFS and BFS. Bidirectional Search Issues in applying: n༆  The predecessor of each node should be efficiently computable. q༇  n༆  n༆  When actions are easily reversible. Goal node: sometimes not unique or known explicitly (e.g., in chess). Memory consumption 12 9/20/13 Comparison of search strategies Criterion BreadthFirst Uniformcost Depth-First Depthlimited Iterative deepening Bidirection al search Complete? YES* YES* NO YES, if l ≥ d YES YES* Time bd+1 bC*/e bm bl bd bd/2 Space bd+1 bC*/e bm bl bd bd/2 Optimal? YES* YES* NO NO YES YES 13...
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This note was uploaded on 02/10/2014 for the course CS 440 taught by Professor Staff during the Fall '08 term at Colorado State.

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