Method of generating search trees and navigation device

A method of generating search trees (25, 27) indicating next valid characters for an input interface of a navigation device includes determining a search sub-tree (29) which indicates next valid characters for both a subset of a first set of character strings and for a different second set of character strings. A first search tree (25) is generated based on information on the first set of character strings, and a second search tree (27) is generated based on information on the second set of character strings. The first search tree (25) is generated such that a node (26) of the first search tree (25) references the search sub-tree (29). The second search tree (27) is generated such that another node (28) of the second search tree (27) references the search sub-tree (29).