new: Solved the first 10 problems

Problems 1-10/problem_1.hs

@@ -2,7 +2,7 @@
 myLast :: [a] -> a
 myLast [] = error "Empty list"
 myLast [s] = s
-myLast (hd : tl) = myLast tl
+myLast (_ : tl) = myLast tl
 
 -- Alternative Solution using fold
 myLast' :: [a] -> a

Problems 1-10/problem_10.hs

@@ -0,0 +1,11 @@
+encode :: (Eq a, Num n) => [a] -> [(n, a)]
+encode =
+  foldr
+    ( \x acc -> case acc of
+        [] -> [(1, x)]
+        (hd@(hd_n, hd_a) : tl) ->
+          if x == hd_a
+            then (hd_n + 1, hd_a) : tl
+            else (1, x) : hd : tl
+    )
+    []

Problems 1-10/problem_2.hs

@@ -0,0 +1,4 @@
+myButLast :: [a] -> a
+myButLast [] = error "List length less than 2"
+myButLast [a, _] = a
+myButLast (hd : tl) = myButLast tl

Problems 1-10/problem_3.hs

@@ -0,0 +1,4 @@
+elementAt :: (Num n, Eq n) => [a] -> n -> a
+elementAt [] _ = error "Invalid index"
+elementAt (hd : _) 1 = hd
+elementAt (_ : tl) k = elementAt tl (k - 1)

Problems 1-10/problem_4.hs

@@ -0,0 +1,8 @@
+-- Simple recursive solution
+myLength :: (Num n) => [a] -> n
+myLength [] = 0
+myLength (_ : tl) = 1 + myLength tl
+
+-- Solution using fold
+myLength' :: (Num n) => [a] -> n
+myLength' = foldl (\acc _ -> acc + 1) 0

Problems 1-10/problem_5.hs

@@ -0,0 +1,2 @@
+myReverse :: [a] -> [a]
+myReverse = foldl (flip (:)) []

Problems 1-10/problem_6.hs

@@ -0,0 +1,2 @@
+isPalindrome :: (Eq a) => [a] -> Bool
+isPalindrome = (==) <$> foldl (flip (:)) [] <*> id

Problems 1-10/problem_7.hs

@@ -0,0 +1,6 @@
+data NestedList a = Elem a | List [NestedList a]
+
+flatten :: NestedList a -> [a]
+flatten x = case x of
+  Elem e -> [e]
+  List l -> concatMap flatten l

Problems 1-10/problem_8.hs

@@ -0,0 +1,10 @@
+compress :: (Eq a) => [a] -> [a]
+compress =
+  foldr
+    ( \x acc ->
+        if acc /= []
+          && x == head acc
+          then acc
+          else x : acc
+    )
+    []

Problems 1-10/problem_9.hs

@@ -0,0 +1,11 @@
+pack :: (Eq a) => [a] -> [[a]]
+pack =
+  foldr
+    ( \x acc -> case acc of
+        [] -> [[x]]
+        (hd : tl) ->
+          if x == head hd
+            then (x : hd) : tl
+            else [x] : hd : tl
+    )
+    []