added __len__ function (#1812)

* added __len__ function

Added a function to count number of nodes in linked list

* Updated __len__ method

used snake_case instead of camel case

* Add tests to __len__()

Co-authored-by: Christian Clauss <cclauss@me.com>

data_structures/linked_list/singly_linked_list.py

@@ -106,6 +106,35 @@ class LinkedList:
                 raise IndexError("Index out of range.")
             current = current.next
         current.data = data
+       
+    def __len__(self):
+        """
+        Return length of linked list i.e. number of nodes
+        >>> linked_list = LinkedList()
+        >>> len(linked_list)
+        0
+        >>> linked_list.insert_tail("head")
+        >>> len(linked_list)
+        1
+        >>> linked_list.insert_head("head")
+        >>> len(linked_list)
+        2
+        >>> _ = linked_list.delete_tail()
+        >>> len(linked_list)
+        1
+        >>> _ = linked_list.delete_head()
+        >>> len(linked_list)
+        0
+        """
+        if not self.head:
+            return 0
+
+        count = 0
+        cur_node = self.head
+        while cur_node.next:
+            count += 1
+            cur_node = cur_node.next
+        return count + 1
 
 
 def main():
@@ -135,6 +164,7 @@ def main():
     A[1] = input("Enter New Value: ").strip()
     print("New list:")
     print(A)
+    print(f"length of A is : {len(A)}")
 
 
 if __name__ == "__main__":