H-99: Ninety-Nine Haskell Problems 2017-4-7

首先附上地址: 99 questions/1 to 10 - HaskellWiki

说明：问题的答案只代表我自己的思路，如果你有更好的解决方法，请告诉我。

Problem 1

序列最后一个元素 Ex: [1,2,3,4] -> 4

myLast :: [a] -> a
myLast []     = error "Empty List"
myLast [x]    = x
myLast (_:xs) = myLast xs 

Problem 2

序列倒数第二个元素 Ex: [1,2,3,4] -> 3

myButLast :: [a] -> a
myButLast []     = error "Empty List"
myButLast [x]    = error "Only One Element"
myButLast [x,_]  = x
myButLast (_:xs) = myButLast xs

Problem 3

序列的第k个元素 Ex: elementAt [1,2,3] 2 -> 2

elementAt :: [a] -> Int -> a
elementAt xs n = xs !! (n-1)

Problem 4

序列的长度 Ex: [1,2,3,4] -> 4

myLength :: [a] -> Int
myLength []     = 0
myLength (_:xs) = 1 + myLength xs

Problem 5

反转序列 Ex: [1,2,3] -> [3,2,1]

myReverse :: [a] -> [a]
myReverse []     = []
myReverse (x:xs) = (reverse xs) ++ [x]

Problem 6

判断是否是一个回文序列 Ex: isPalindrome [1,2,3] -> False; isPalindrome "madamimadam" -> True; isPalindrome [1,2,4,8,16,8,4,2,1] -> True

isPalindrome :: (Eq a) => [a] -> Bool
isPalindrome xs = if (length xs) `mod` 2 == 0
                  then False
                  else xs == reverse xs