利用模式匹配和递归计算list

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本文介绍使用Scala实现树形结构的递归遍历，包括求解叶子节点之和及表达式树的计算结果。文章通过具体示例展示了如何定义树节点与叶子节点，并实现递归函数来处理这些树形结构。

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val l = List(List(9,8),8,List(12))

 def leafSum(l: List[Any]): Int = l.map(_ match {
   case l: List[Any] => leafSum(l)
   case x: Int => x
   case _ => 0
 }).sum

 println(leafSum(l))

/****************************************/

//使用新方法实现

sealed abstract class BinaryTree
case class Leaf(value: Int) extends BinaryTree
case class Node(left:BinaryTree,right: BinaryTree) extends BinaryTree

val n = Node(Node(Leaf(3),Leaf(8)),Leaf(5))

def leafSum(bt:BinaryTree):Int = bt match {
  case Node(left,right) => leafSum(left) + leafSum(right)
  case Leaf(value) => value
}

print( leafSum(n) == 16)
/*******************************/
//扩展使每个节点可以有任意多的后代
sealed abstract class BinaryTree
case class Leaf(value: Int) extends BinaryTree
case class Node(bt:BinaryTree*) extends BinaryTree

val n = Node(Node(Leaf(3),Leaf(5)),Leaf(2),Node(Leaf(5)))

def leafSum(bt:BinaryTree):Int = bt match {
  case Node(bts @ _*) => bts.map(leafSum).sum
  case Leaf(value) => value
}
print( leafSum(n))

/********************************************/
//每个非叶子节点存放操作符
sealed abstract class BinaryTree
case class Leaf(value: Int) extends BinaryTree
case class Node(operator: Char,bt:BinaryTree*) extends BinaryTree

val n = Node('+', Node('*', Leaf(3), Leaf(8)), Leaf(2), Node('-', Leaf(5)))

def eval(bt:BinaryTree):Int = bt match {
  case Node('+',bts @ _*) => bts.map(eval).sum
  case Node('-',bts @ _*) => bts.map(eval).foldLeft(0)(_ - _)
  case Node('*',bts @ _*) => bts.map(eval).product
  case Leaf(value) => value
  case _ => 0
}
print( eval(n))