merge two map and sum its values
多个map合并,key相同时则value相加
val map1 = Map(1 -> 1, 2 -> 2)
val map2 = Map(1 -> 11, 3 -> 3)
val map3 = Map(1 -> 111, 3 -> 3)
val mapList = List(map1, map2, map3)
val merged = mapList.reduce((m1, m2) =>
m1 ++ m2.map { case (k, v) => k -> (v + m1.getOrElse(k, 0)) }
)