package tree object SizeByFold { def size[A](t: Tree[A]) = Fold.fold(t)(_ => 1)(_ + _ + 1) def main(args: Array[String]): Unit = { val tree = Branch(Leaf(1), Branch(Branch(Branch(Leaf(3), Branch(Leaf(5), Leaf(6))), Leaf(4)), Leaf(2))) println(size(tree)) } }
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原文：http://www.cnblogs.com/JonkeyGuan/p/5424831.html