Returns a RDD from grouping items of its parent RDD in fixed size blocks by passing a sliding
window over them. The ordering is first based on the partition index and then the ordering of
items within each partition. This is similar to sliding in Scala collections, except that it
becomes an empty RDD if the window size is greater than the total number of items. It needs to
trigger a Spark job if the parent RDD has more than one partitions and the window size is
greater than 1.
public T treeReduce(scala.Function2<T,T,T> f,
Reduces the elements of this RDD in a multi-level tree pattern.