Scala中硬币变化的StackOverflowError? [英] StackOverflowError for coin change in Scala?

问题描述

我正在为 Scala 中的硬币(找零)问题编写递归函数.

I'm writing a recursive function for the Coin (change) problem in Scala.

我的实现因 StackOverflowError 而中断，我无法弄清楚为什么会发生这种情况.

My implementation breaks with StackOverflowError and I can't figure out why it happens.

Exception in thread "main" java.lang.StackOverflowError
    at scala.collection.immutable.$colon$colon.tail(List.scala:358)
    at scala.collection.immutable.$colon$colon.tail(List.scala:356)
    at recfun.Main$.recurs$1(Main.scala:58) // repeat this line over and over

这是我的电话:

  println(countChange(20, List(1,5,10)))

这是我的定义:

def countChange(money: Int, coins: List[Int]): Int =  {

  def recurs(money: Int, coins: List[Int], combos: Int): Int = 
  {    
      if (coins.isEmpty)
          combos
      else if (money==0)
          combos + 1
      else
          recurs(money,coins.tail,combos+1) + recurs(money-coins.head,coins,combos+1)

  }
  recurs(money, coins, 0)
} 

我刚刚在混合中添加了 else if 语句:

I just added the else if statement in the mix:

else if(money<0)
    combos

它摆脱了错误，但我的输出是 1500 东西 :( 我的逻辑有什么问题?

it got rid of the error but my output is 1500 something :( what is wrong with my logic?

推荐答案

以下是基于您的代码的正确解决方案:

Here is the correct solution based on your codes:

def countChange(money: Int, coins: List[Int]): Int = {
  def recurs(m: Int, cs: List[Int], cnt: Int): Int =
      if(m < 0) cnt  //Not a change, keep cnt
      else if(cs.isEmpty) {
        if(m == 0) cnt + 1 else cnt // plus cnt if find a change
      }
      else recurs(m, cs.tail, cnt) + recurs(m-cs.head, cs, cnt)
  recurs(money, coins, 0)
}

总之，有一个简短的解决方案(但效率不高，您可以缓存中间结果以使其高效.)

Anyway, there is a short solution(But not efficient, you can cache the middle result to make it efficient.)

def countChange(m: Int, cs: List[Int]): Int = cs match {
  case Nil => if(m == 0) 1 else 0
  case c::rs => (0 to m/c) map (k => countChange(m-k*c,rs)) sum
}

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