Branches - Linnea

lib/recursive-methods.rb

@@ -1,15 +1,25 @@
 # Authoring recursive algorithms. Add comments including time and space complexity for each method.
 
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n)
+# Space complexity: O(n)
 def factorial(n)
-    raise NotImplementedError, "Method not implemented"
+  if n < 0
+    raise ArgumentError
+  elsif n == 0 || n == 1
+    return 1
+  else
+    return n * (factorial(n - 1))
+  end
 end
 
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n)
+# Space complexity: O(n)
 def reverse(s)
-    raise NotImplementedError, "Method not implemented"
+  if s.length <= 1
+    return s
+  else
+    return reverse(s[1..-1]) + s[0]
+  end
 end
 
 # Time complexity: ?
@@ -36,10 +46,18 @@ def search(array, value)
     raise NotImplementedError, "Method not implemented"
 end
 
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n)
+# Space complexity: O(n)
 def is_palindrome(s)
-    raise NotImplementedError, "Method not implemented"
+  if s.length == 1 || s.length == 0
+    return true
+  end
+
+  if s[0] != s[-1]
+    return false
+  end
+
+  return is_palindrome(s[1...-1])
 end
 
 # Time complexity: ?

test/recursion_writing_test.rb

@@ -38,7 +38,7 @@
   end
 end
 
-xdescribe "reverse" do
+describe "reverse" do
   it "will reverse 'cat'" do
     # Arrange
     string = "cat"
@@ -260,7 +260,7 @@
     end      
 end
 
-xdescribe "is_palindrome" do
+describe "is_palindrome" do
   it "will return true for emptystring" do
     # Arrange
     string = ""