erlang的非平衡的二叉树的操作

-module(tree1).

-export([test1/0]).

lookup(Key,nil)    ->
    not_found;
lookup(Key,{Key,Value,_,_})    ->
    {found,Value};
lookup(Key,{Key1,_,Smaller,_}) when Key < Key1    ->
    lookup(Key,Smaller);
lookup(Key,{Key1,_,_,Bigger})    when Key > Key1 ->
    lookup(Key,Bigger).

insert(Key,Value ,nil)    ->
    {Key,Value,nil,nil};
insert(Key,Value,{Key,_,Smaller,Bigger})    ->
    {Key,Value,Smaller,Bigger};
insert(Key,Value,{Key1,V,Smaller,Bigger})    when Key < Key1 ->
    {Key1,V,insert(Key,Value,Smaller),Bigger};
insert(Key,Value,{Key1,V,Smaller,Bigger})    when Key > Key1    ->
    {Key1,V,Smaller,insert(Key,Value,Bigger)}.
write_tree(T)    ->
    write_tree(0,T).
write_tree(D,nil)    ->
%    io:tab(D),
    io:format('nil',[]);
write_tree(D,{Key,Value,Smaller,Bigger})    ->
    D1 = D +4 ,
    write_tree(D1,Bigger),
    io:format('~n',[]),
    %io:tab(D),
    io:format('~w ==> ~w~n',[Key,Value]),
    write_tree(D1,Smaller).
test1()    ->
    S1=nil,
    S2=insert(4,joe,S1),
    S3=insert(10,fred,S2),
    S4=insert(3,jane,S3),
    S5=insert(7,kalle,S4),
    S6=insert(6,thomes,S5),
    S7=insert(5,rickard,S6),
    S8=insert(9,susan,S7),
    S9=insert(2,tobbe,S8),
    S10=insert(8,dan,S9),
    write_tree(S10).