合并排序不适用于升序答案

【问题标题】：Merge sort not working for ascending order合并排序不适用于升序
【发布时间】：2019-09-12 20:39:51
【问题描述】：

我正在学习一门关于 scala 的课程，其中我正在执行合并排序操作。这是为此编写的代码

def merge(leftList:List[Int], rightList:List[Int]): List[Int] = {

  val output = (0 until leftList.length + rightList.length).foldLeft(List[Int](), leftList, rightList) { (triple, _) =>
    val (mergedList, leftRemaining, rightRemaining) = triple
    (leftRemaining, rightRemaining) match {
      case (Nil, r :: rTail) => (r :: mergedList, Nil, rTail)
      case (l :: lTail, Nil) => (l :: mergedList, lTail, Nil)
      case (l :: lTail, r :: rTail) if l > r => (l :: mergedList, lTail, rightRemaining)
      case (l :: lTail, r :: rTail) => (r :: mergedList, leftRemaining, rTail)
    }
  }
  output._1.reverse
}

def mergeSort(inputList: IndexedSeq[Int]): List[Int] = {
  if(inputList.length == 1) List(inputList.head)
  else {
    val (leftList, rightList) = inputList.splitAt(inputList.length / 2)
    val sortedLeft = mergeSort(leftList)
    val sortedRight = mergeSort(rightList)
    merge(sortedLeft, sortedRight)
  }
}

mergeSort(Vector(3,4,1,2,33))

但是，列表不是按升序排序，而是始终按降序排序。我尝试调试代码，但没有得出任何结论。任何帮助将不胜感激。我目前的输出如下

res0: List[Int] = List(33, 4, 3, 2, 1)

预期输出是

List(1, 2, 3, 4, 33)

【问题讨论】：

将 .reverse 改为 sorted

标签： list algorithm scala math functional-programming

【解决方案1】：

因为这里：

case (l :: lTail, r :: rTail) if l > r => (l :: mergedList, lTail, rightRemaining)

如果l 大于r，则您将放在第一位。如果要升序，则应将其反转：

case (l :: lTail, r :: rTail) if l <= r => (l :: mergedList, lTail, rightRemaining)

【讨论】：

【解决方案2】：

解决方案非常简单。您只需将 (l > r) 更改为 (l

请参考以下代码

def merge(leftList:List[Int], rightList:List[Int]): List[Int] = {

  val output = (0 until leftList.length + rightList.length).foldLeft(List[Int](), leftList, rightList) { (triple, _) =>
    val (mergedList, leftRemaining, rightRemaining) = triple
    (leftRemaining, rightRemaining) match {
      case (Nil, r :: rTail) => (r :: mergedList, Nil, rTail)
      case (l :: lTail, Nil) => (l :: mergedList, lTail, Nil)
      case (l :: lTail, r :: rTail) if l < r => (l :: mergedList, lTail, rightRemaining)
      case (l :: lTail, r :: rTail) => (r :: mergedList, leftRemaining, rTail)
    }
  }
  output._1.reverse
}

def mergeSort(inputList: IndexedSeq[Int]): List[Int] = {
  if(inputList.length == 1) List(inputList.head)
  else {
    val (leftList, rightList) = inputList.splitAt(inputList.length / 2)
    val sortedLeft = mergeSort(leftList)
    val sortedRight = mergeSort(rightList)
    merge(sortedLeft, sortedRight)
  }
}

这会产生所需格式的输出

res0: List[Int] = List(1, 2, 3, 4, 33)

如果这回答了您的问题，请告诉我。

【讨论】：