change space complexity of linked list's __len__ from O(n) to O(1) (#8183)

data_structures/linked_list/circular_linked_list.py

@@ -24,7 +24,7 @@ class CircularLinkedList:
                 break
 
     def __len__(self) -> int:
-        return len(tuple(iter(self)))
+        return sum(1 for _ in self)
 
     def __repr__(self):
         return "->".join(str(item) for item in iter(self))

data_structures/linked_list/doubly_linked_list.py

@@ -51,7 +51,7 @@ class DoublyLinkedList:
         >>> len(linked_list) == 5
         True
         """
-        return len(tuple(iter(self)))
+        return sum(1 for _ in self)
 
     def insert_at_head(self, data):
         self.insert_at_nth(0, data)

data_structures/linked_list/merge_two_lists.py

@@ -44,7 +44,7 @@ class SortedLinkedList:
         >>> len(SortedLinkedList(test_data_odd))
         8
         """
-        return len(tuple(iter(self)))
+        return sum(1 for _ in self)
 
     def __str__(self) -> str:
         """

data_structures/linked_list/singly_linked_list.py

@@ -72,7 +72,7 @@ class LinkedList:
         >>> len(linked_list)
         0
         """
-        return len(tuple(iter(self)))
+        return sum(1 for _ in self)
 
     def __repr__(self) -> str:
         """