Haskell --- 实现快读排序

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类型声明

quicksort :: (Ord a) => [a] -> [a]

边界条件：

空List。排过序的空List还是空List。接下来便是算法的定义：排过序的List就是令所有小于等于头部的元素在先(它们已经排过了序), 后跟大于头部的元素(它们同样已经拍过了序)。

quicksort :: (Ord a) => [a] -> [a]  
quicksort [] = []  
quicksort (x:xs) =  
  let smallerSorted = quicksort [a | a <- xs, a <= x] biggerSorted = quicksort [a | a <- xs, a > x]  
  in smallerSorted ++ [x] ++ biggerSorted