datatype ball = Void | Bt of int*ball*ball ;

val Balltree =(Bt(3,Bt(1,Bt(0,Void,Void),Bt(2,Void,Void)),Bt(7,Bt(5,Bt(4,Void,Void),Bt(6,Void,Void)),Bt(8,Void,Void))));


fun preorder Void = []
  | preorder (Bt(r,L,R)) = r ::(preorder L ) @ (preorder R);

fun compare (Void,st) = st
  | compare (Bt(r,L1,R1),st) = Bt(r,L1,compare(R1,st));


fun delete (Void,k) = Void
  | delete (Bt(r,L,R),k) = if r = k then
          compare(L,R)
      else 
          Bt(r,delete(L,k),delete(R,k));