Source code in the swift programming language

import Foundation

func isBal(str: String) -> Bool {
  
  var count = 0
  
  return !str.characters.contains { ($0 == "["  ? ++count : --count) < 0 } && count == 0
  
}

isBal("[[[]]]") // true

isBal("[]][[]") // false

func randBrack(n: Int) -> String {
  
  var bracks: [Character] = Array(Repeat(count: n, repeatedValue: "["))
  
  for i in UInt32(n+1)...UInt32(n + n) {
    
    bracks.insert("]", atIndex: Int(arc4random_uniform(i)))
    
  }
  
  return String(bracks)
  
}

randBrack(2) // "]][["

func randIsBal(n: Int) {
  
  let (bal, un) = ("", "un")
  
  for str in (1...n).map(randBrack) {
    
    print("\(str) is \(isBal(str) ? bal : un)balanced\n")
    
  }
}

randIsBal(4)

//    ][ is unbalanced
//
//    ]][[ is unbalanced
//
//    []][[] is unbalanced
//
//    [][][[]] is balanced