Github user sethah commented on a diff in the pull request:
https://github.com/apache/spark/pull/20632#discussion_r169737984
--- Diff: mllib/src/main/scala/org/apache/spark/ml/tree/Node.scala ---
@@ -287,6 +291,34 @@ private[tree] class LearningNode(
}
}
+ /**
+ * @return true iff the node is a leaf.
+ */
+ private def isLeafNode(): Boolean = leftChild.isEmpty &&
rightChild.isEmpty
+
+ // the set of (leaf) predictions appearing in the subtree rooted at the
given node.
+ private lazy val leafPredictions: Set[Double] = {
--- End diff --
This is a recursive method. It essentially does this:
```scala
def prune(node: Node): Node = {
val left = prune(node.left)
val right = prune(node.right)
if (left.isLeaf && right.isLeaf && left.prediction == right.prediction) {
new LeafNode
} else {
new InternalNode
}
}
```
It starts pruning at the bottom and then works its way up.
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