A metric between unrooted and unordered trees and its bottom-up computing method

Proposes a distance measure between unrooted and unordered trees based on the strongly structure-preserving mapping (SSPM). SSPM can make correspondences between the vertices of similar substructures of given structures more strictly than previously proposed mappings. The time complexity of computing the distance between trees T_a and T _b is O(m_b³N_aN_b), where N_a and N_b are the number of vertices in trees T_a and T_b, respectively; m_a and m _b are the maximum degrees of a vertex in T_a and T _b, respectively; and m_a⩽m_b is assumed. The space complexity of the method is O(N_aN_b )