[Feature] Reversing Linked Lists iteratively and recursively!

Author: exterkamp

structures/linked_list.py

@@ -39,8 +39,33 @@ def get_beginning_of_cycle_if_exists(self):
         slow_pointer = self
         # move the pointers again at same speed, will meet at start
         while slow_pointer is not fast_pointer:
-            print(slow_pointer.val, fast_pointer.val)
+            # print(slow_pointer.val, fast_pointer.val)
             slow_pointer = slow_pointer.next
             fast_pointer = fast_pointer.next
 
-        return slow_pointer
+        return slow_pointer
+
+    def reverse(self):
+        prev = None
+        head = self
+        while head.next:
+            next_node = head.next
+            head.next = prev
+            prev = head
+            head = next_node
+        head.next = prev
+        return head
+
+    def reverse_recursive(self):
+        def rev(node):
+            if not node.next:
+                rev.head = node
+                return
+            rev(node.next)
+            temp = node.next
+            temp.next = node
+            node.next = None
+        rev.head = self
+        rev(self)
+
+        return rev.head

tests/test_linked_list.py

@@ -53,4 +53,28 @@ def test_ll_get_cycle_start_no_cycle(self):
         self.assertEqual(None, head.get_beginning_of_cycle_if_exists())
 
     def test_ll_get_cycle_start_on_single_node(self):
-        self.assertEqual(None, ListNode(1).get_beginning_of_cycle_if_exists())
+        self.assertEqual(None, ListNode(1).get_beginning_of_cycle_if_exists())
+    
+    def test_reverse(self):
+        head = ListNode(1)
+        head.next = ListNode(2)
+        head.next.next = ListNode(3)
+        self.assertEqual(1, head.val)
+        self.assertEqual(2, head.next.val)
+        self.assertEqual(3, head.next.next.val)
+        head = head.reverse()
+        self.assertEqual(3, head.val)
+        self.assertEqual(2, head.next.val)
+        self.assertEqual(1, head.next.next.val)
+    
+    def test_reverse_recursive(self):
+        head = ListNode(1)
+        head.next = ListNode(2)
+        head.next.next = ListNode(3)
+        self.assertEqual(1, head.val)
+        self.assertEqual(2, head.next.val)
+        self.assertEqual(3, head.next.next.val)
+        head = head.reverse_recursive()
+        self.assertEqual(3, head.val)
+        self.assertEqual(2, head.next.val)
+        self.assertEqual(1, head.next.next.val)