val map1: mutable.Map[String, Int] = mutable.Map("a" -> 1, "b" -> 2, "c" -> 3)
    val map2: mutable.Map[String, Int] = mutable.Map("a" -> 2, "b" -> 4, "d" -> 3)
    map1.foldLeft(map2){
      (mm,kv)=>{
        val k: String = kv._1
        val v: Int = kv._2
        mm(k)=mm.getOrElse(k,0)+v
        mm
      }
    }
    println(map1)
    println(map2)
//output 
Map(b -> 2, a -> 1, c -> 3)
Map(b -> 6, d -> 3, a -> 3, c -> 3)