1. A graph search method comprising: generating an ordered decision tree structure to represent the set of search frontier nodes with internal nodes of the decision tree corresponding to decision nodes operative to perform tests on state variables, and the leaf nodes corresponding to nodes in a search frontier; performing depth-first traversal in the ordered decision tree structure to order node expansions such that a node to be expanded next is a close neighbor of a current node; and, deriving an intermediate graph of the selected node by modifying the intermediate graph of the current node.