private def subtraction(v1: String, v2: String): String = {
    var number = v2.toCharArray.map(_.asDigit)
    var thisNumber = v1.toCharArray.map(_.asDigit)
    var result = ListBuffer[Int]()
    var negative = false

    if(number.length > thisNumber.length || number.length == thisNumber.length && number(0) > thisNumber(0)) {
      negative = true
      var tmp = thisNumber
      thisNumber = number
      number = tmp
    }

    val sup: Int => Unit = (i: Int) => {
      thisNumber(i) match {
        case j if j > 0 =>
          thisNumber.update(i, j-1)
        case _ =>
          thisNumber.update(i, 9)
          sup(i-1)
      }
    }

    val minus = (i: Int, j: Int) => {
      val res = thisNumber(i) - number(j)
      res match {
        case t if t >= 0 => thisNumber.update(i, t)
        case _ =>
          sup(i-1)
          thisNumber.update(i, res + 10)
      }
    }

    for(i <- 0 until Math.min(thisNumber.length, number.length)) {
      minus(thisNumber.length - i - 1, number.length - i - 1)
    }
    clear((if(negative)"-") + thisNumber.mkString("","",""))
  }