原型：
def reduceByKeyLocally(func: (V, V) => V): Map[K, V]
该函数将RDD[K,V]中每个K对应的V值根据映射函数来运算，运算结果映射到一个Map[K,V]中，而不是RDD[K,V]
scala> var rdd1 = sc.makeRDD(Array(("A",0),("A",2),("B",1),("B",2),("C",1)))
rdd1: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[91] at makeRDD at :21
scala> rdd1.reduceByKeyLocally((x,y) => x + y)
res90: scala.collection.Map[String,Int] = Map(B -> 3, A -> 2, C -> 1)