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



balanced <- function(str){
  str <- strsplit(str, "")[[1]]
  str <- ifelse(str=='[', 1, -1)
  all(cumsum(str) >= 0) && sum(str) == 0
}