def mingleStrings(s1:String,s2:String) :String =  {
        val n = s1.length()
        val sb = new StringBuilder("")
        def build(i:Int,accumulator:sb.type) :String = {
            if(i==n){
                return accumulator.toString
            }
            accumulator+=s1.charAt(i)
            accumulator+=s2.charAt(i)
            return build(i+1,accumulator)
        }
        return build(0,sb) 
    }