从预订和中间列表重建二叉树

问题描述

你好我想重建一个二叉树，我几乎得到它，只是它抛出一个错误，我不知道为什么

  buildTree ::（Ord a，Eq a）=> [a]  - > [a]  - >树a 
 buildTree [] [] =空
 buildTree preOrd inOrd =节点root左右
其中root = head preOrd 
 left = buildTree leftPreOrd leftInOrd 
 right = buildTree rigthPreOrd leftInOrd 
 
 Just rootInd = elemIndex root inOrd 
 leftPreOrd = tail（take（rootInd + 1）preOrd）
 rigthPreOrd = tail（drop rootInd preOrd）
 
 leftInOrd = take rootInd inOrd 
 rightInord = drop（rootInd + 1）inOrd 
  

当我打电话给它使用

  buildTree [10,5,2,6,14,12,15] [2,5,6 ，10,12,14,15] 
  

它引发了我：

 例外情况：reconstruir.hs：26：11-45：不可模仿的模式失败只需rootInd 
  

解决方案

运行时在这条线上失败：

 只需rootInd = elemIndex root inOrd 
  

elemI ndex 在运行示例输入时返回 Nothing ，但是您的代码表示它将始终返回 Just ，所以运行时崩溃。您需要处理 elemIndex root inOrd 返回 Nothing 。

也许更重要的是，您应该使用显示为编译器错误，以便您的代码无法编译以开始。

Hi I'm trying to rebuild a binary tree, I almost got it, except it throws me an error and I don't know why

buildTree :: (Ord a, Eq a) => [a] -> [a] -> Tree a
buildTree [] [] = Empty
buildTree preOrd inOrd = Node root left right
where root  = head preOrd
      left = buildTree leftPreOrd leftInOrd
      right = buildTree rigthPreOrd leftInOrd

      Just rootInd = elemIndex root inOrd
      leftPreOrd   = tail (take (rootInd + 1) preOrd)
      rigthPreOrd  = tail (drop rootInd preOrd)

      leftInOrd    = take rootInd inOrd
      rightInord   = drop (rootInd + 1) inOrd

When I call it using

buildTree [10,5,2,6,14,12,15] [2,5,6,10,12,14,15]

it throws me this:

Exception: reconstruir.hs:26:11-45: Irrefutable pattern failed for pattern Just rootInd

解决方案

The runtime is failing on this line:

Just rootInd = elemIndex root inOrd

elemIndex is returning Nothing when running your example input, but your code says it will always return a Just, so the runtime crashes. You need to handle the case where elemIndex root inOrd returns Nothing.

Perhaps more importantly, you should enable all warnings with the -Wall flag to show up as compiler errors so that your code wouldn't compile to begin with.

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