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



 
from random import random, randomize, shuffle
from strutils import repeat
 
randomize()
 
proc gen(n: int): string =
  result = "[]".repeat(n)
  shuffle(result)
 
proc balanced(txt: string): bool =
  var b = 0
  for c in txt:
    case c
    of '[':
      inc(b)
    of ']':
      dec(b)
      if b < 0: return false
    else: discard
  b == 0
 
for n in 0..9:
  let s = gen(n)
  echo "'", s, "' is ", (if balanced(s): "balanced" else: "not balanced")