list - 删除重复但保留顺序

rmdup :: [Int] -> [Int]
rmdup [] = []
rmdup (x:xs) | x `elem` xs = rmdup xs
             | otherwise = x: rmdup xs

上面的代码从“整数”列表中删除重复项，但删除了第一个匹配项，并保留了第二个匹配项。例如:

rmdup [1,2,3,1,4]

将导致:

[2,3,1,4]

我如何更改它以保持顺序并产生此结果:[1,2,3,4]？ 注意，我不想使用内置功能。

最佳答案

接下来呢？这避免了疯狂低效的acc ++ [x]，并且还反转了给定列表两次:

rmdup :: Eq a => [a] => [a]
rmdup xs = rmdup' [] xs
  where
    rmdup' acc [] = []
    rmdup' acc (x:xs)
      | x `elem` acc = rmdup' acc xs
      | otherwise    = x : rmdup' (x:acc) xs