-- 寫一個函數(shù) lastButOne, 返回列表倒數(shù)第二個元素.
lastButOne [] = error "error"
lastButOne (x: []) = error "error"
lastButOne (x: y: []) = x
lastButOne (x: y: ys) = lastButOne $ y: ys
-- 寫一個函數(shù)柳击，用來計算一個列表元素的個數(shù)．出于測試要求，保證其輸出的結(jié)果和標(biāo)準(zhǔn)函數(shù) length 保持一致．
length' [] = 0
length' (x: xs) = 1 + length' xs
-- 添加函數(shù)的類型簽名于你的源文件．出于測試要求缺猛，再次加載源文件到ghci．
myLength :: [a] -> Int
myLength [] = 0
myLength (x: xs) = 1 + myLength xs
-- 寫一個函數(shù)中姜，用來計算列表的平均值，即薯演，列表元素的總和除以列表的長度．(你可能需要用到 fromIntegral 函數(shù)將列表長度變量從 integer 類型到 float 類型進(jìn)行轉(zhuǎn)換．)
avg :: (Fractional a) => [a] -> a
avg [] = 0
avg xs = (sum xs) / (fromIntegral $ length xs)
-- 將一個列表變成回文序列撞芍，即，他應(yīng)該讀起來完全一樣跨扮，不管是從前往后還是從后往前．舉個例子序无，考慮一個列表 [1,2,3]，你的函數(shù)應(yīng)該返回 [1,2,3,3,2,1]．
reverseArray :: [a] -> [a]
reverseArray [] = []
reverseArray xs = xs ++ reverse' xs
reverse' :: [a] -> [a]
reverse' [] = []
reverse' (x: xs) = reverse' xs ++ [x]
-- 寫一個函數(shù)衡创，用來確定他的輸入是否是一個回文序列．
isPali :: (Eq a) => [a] -> Bool
isPali [] = True
isPali xs
| reverse' xs == xs = True
| otherwise = False
-- 創(chuàng)造一個函數(shù)帝嗡，用于排序一個包含許多列表的列表，其排序規(guī)則基于他的子列表的長度．（你可能要看看 Data.List 模塊的 sortBy 函數(shù)．）
sortByLength :: (Ord a) => [[a]] -> [[a]]
sortByLength xs = sortBy compareLength xs
where compareLength x y = compare (length x) (length y)
-- 定義一個函數(shù)璃氢，其用一個分隔符將一個包含許多列表的列表連接在一起．函數(shù)類型定義如下：
intersperse' :: a -> [[a]] -> [a]
intersperse' _ [x] = x
intersperse' seq (x: xs) = x ++ [seq] ++ (intersperse' seq xs)
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