Task:

 Generate a string with   N   opening brackets   [   and with   N   closing brackets   ],   in some arbitrary order. 
 Determine whether the generated string is balanced; that is, whether it consists entirely of pairs of opening/closing brackets (in that order), none of which mis-nest.


Examples

   (empty)      OK
   []           OK   
   [][]         OK   
   [[][]]       OK 
   ][         NOT OK
   ][][       NOT OK
   []][[]     NOT OK



fun isBalanced s = checkBrackets 0 (String.explode s)
and checkBrackets 0 [] = true
  | checkBrackets _ [] = false
  | checkBrackets ~1 _ = false
  | checkBrackets counter (#"["::rest) = checkBrackets (counter + 1) rest
  | checkBrackets counter (#"]"::rest) = checkBrackets (counter - 1) rest
  | checkBrackets counter (_::rest) = checkBrackets counter rest