datatype 'a Tree = Empty | Node of ('a * 'a Tree * 'a Tree); 

fun findDepth((Node(a,Empty,Empty))) =1 |
    findDepth((Node(a,sol,Empty))) = 1+findDepth(sol) |
    findDepth((Node(a,Empty,sag))) = 1+findDepth(sag) |
   findDepth((Node(a,sol,sag)))=1+let 
				val k=findDepth(sol);
				val l=findDepth(sag)
				in
				  if k>l then k else l
				end; 

fun getChars((Node(a,sol,sag)),all,curr) = if all=curr+1 then [a]
   else if sol=Empty andalso sag=Empty then nil
        else if sag=Empty then getChars(sol,all,curr+1)
             else if sol=Empty then getChars(sag,all,curr+1)
                  else getChars(sol,all,curr+1)@getChars(sag,all,curr+1);  

fun writeLevels(mytree,alldepth,currentdepth) =
  if currentdepth>alldepth then nil
     else [getChars(mytree,currentdepth,0)]@writeLevels(mytree,alldepth,currentdepth+1);

fun getLevels mytree = writeLevels(mytree,findDepth(mytree),1);