% P70 (**) Multiway tree construction from a node string % We suppose that the nodes of a multiway tree contain single % characters. In the depth-first order sequence of its nodes, a % special character ^ has been inserted whenever, during the % tree traversal, the move is a backtrack to the previous level. % Define the syntax of the string and write a predicate tree(String,Tree) % to construct the Tree when the String is given. Work with atoms (instead % of strings). Make your predicate work in both directions. % % Syntax in BNF: % ::= '^' % ::= | % First a nice solution using difference lists tree(TS,T) :- atom(TS), !, atom_chars(TS,TL), tree_d(TL-[],T). % (+,?) tree(TS,T) :- nonvar(T), tree_d(TL-[],T), atom_chars(TS,TL). % (?,+) tree_d([X|F1]-T, t(X,F)) :- forest_d(F1-['^'|T],F). forest_d(F-F,[]). forest_d(F1-F3,[T|F]) :- tree_d(F1-F2,T), forest_d(F2-F3,F). % Another solution, not as elegant as the previous one. tree_2(TS,T) :- atom(TS), !, atom_chars(TS,TL), tree_a(TL,T). % (+,?) tree_2(TS,T) :- nonvar(T), tree_a(TL,T), atom_chars(TS,TL). % (?,+) tree_a(TL,t(X,F)) :- append([X],FL,L1), append(L1,['^'],TL), forest_a(FL,F). forest_a([],[]). forest_a(FL,[T|Ts]) :- append(TL,TsL,FL), tree_a(TL,T), forest_a(TsL,Ts).