Very efficient implementation that combines the values of a RDD consisting of two-component tuples by applying multiple aggregators one after another.
函数原型：
def combineByKeyC: RDD[(K, C)] def combineByKeyC: RDD[(K, C)] def combineByKeyC: RDD[(K, C)]
例子：
val a = sc.parallelize(List("dog","cat","gnu","salmon","rabbit","turkey","wolf","bear","bee"), 3)
val b = sc.parallelize(List(1,1,2,2,2,1,2,2,2), 3)
val c = b.zip(a)
// 第一个函数对每一个key，用来将key的第一个value初始化到一个特定的结构
// 第二个函数用来在每一个partition中进行对初始化结构和values中的每一个item进行reduce函数运算
// 第三个函数用来在对每一个partition上产生的结果按照相同的key再次按照第三个reduce函数进行计算
val d = c.combineByKey(List(_), (x:List[String], y:String) => y :: x, (x:List[String], y:List[String]) => x ::: y)
d.collect
res16: Array[(Int, List[String])] = Array((1,List(cat, dog, turkey)), (2,List(gnu, rabbit, salmon, bee, bear, wolf)))