Question

我正在通过“Scala for the Impatient”一书中的练习来学习Scala。其中一个问题是：

/**
   * Q5: One can use lists to model trees that store values only in the leaves. For example, the list ((3, 8) 2 (5))
   * describes the tree
   *      .
   *    . 2   .
   *  3   8  5
   *
   *  However, some of the list elements are numbers and others are lists. In Scala, you cannot have heterogeneous lists,
   *  so you have to use a `List[Any]`. Write a `leafSum` function to compute the sum of all elements in the leaves,
   *  using pattern matching to differentiate between numbers and lists.
   *
   */

我的代码：

def leafSum(lst: List[Any]): Int = {
  type Node = Tuple2[_, List[Any]]
  type Leaf = Tuple2[Int, Int]

  lst.foldLeft(0) {
    case (_, elem): Node => leafSum(elem)
    case (_, _): Leaf => _ + _
  }
}

但是，两个case语句都无法编译，并出现以下错误。为什么呢？

类型不匹配;发现：所需单位：Int

修改：我知道我可以使用collect，但我希望foldLeft能够完成这笔费用而不必自己动手。

lst.collect {
  case node: List[Any] => leafSum2(node)
  case leaf: Int => leaf
}.sum

EDIT2 ：请参阅下面的解决方案。

Answer 1

模式匹配需要一些调整。如果有效，请告诉我。

if ((amount >= 0) && (amount <= 2)) System.out.println(array[amount]);

Answer 2

这很有效。 @marios的回答让我走上了正确的道路，尽管他的实现有一个错误但没有用。

def leafSum(lst: List[Any]): Int = {
  lst.foldLeft(0) {
    case (sum, node: List[_]) => sum + leafSum(node)
    case (sum, leaf: Int) => sum + leaf
  }
}

使用模式匹配的树叶中所有元素的Scala总和

2 个答案: