Fix mypy errors for data_structures->linked_list directory files (#4927)

data_structures/linked_list/__init__.py

@@ -6,7 +6,7 @@ Nodes contain data and also may link to other nodes:
     - Last node: points to null
 """
 
-from typing import Any
+from typing import Any, Optional
 
 
 class Node:
@@ -17,7 +17,7 @@ class Node:
 
 class LinkedList:
     def __init__(self) -> None:
-        self.head = None
+        self.head: Optional[Node] = None
         self.size = 0
 
     def add(self, item: Any) -> None:
@@ -25,7 +25,10 @@ class LinkedList:
         self.size += 1
 
     def remove(self) -> Any:
-        if self.is_empty():
+        # Switched 'self.is_empty()' to 'self.head is None'
+        # because mypy was considering the possibility that 'self.head'
+        # can be None in below else part and giving error
+        if self.head is None:
             return None
         else:
             item = self.head.item
@@ -50,7 +53,7 @@ class LinkedList:
         else:
             iterate = self.head
             item_str = ""
-            item_list = []
+            item_list: list[str] = []
             while iterate:
                 item_list.append(str(iterate.item))
                 iterate = iterate.next

data_structures/linked_list/circular_linked_list.py

@@ -1,10 +1,10 @@
-from typing import Any
+from typing import Any, Iterator, Optional
 
 
 class Node:
     def __init__(self, data: Any):
-        self.data = data
-        self.next = None
+        self.data: Any = data
+        self.next: Optional[Node] = None
 
 
 class CircularLinkedList:
@@ -12,7 +12,7 @@ class CircularLinkedList:
         self.head = None
         self.tail = None
 
-    def __iter__(self):
+    def __iter__(self) -> Iterator[Any]:
         node = self.head
         while self.head:
             yield node.data
@@ -54,10 +54,10 @@ class CircularLinkedList:
     def delete_front(self):
         return self.delete_nth(0)
 
-    def delete_tail(self) -> None:
+    def delete_tail(self) -> Any:
         return self.delete_nth(len(self) - 1)
 
-    def delete_nth(self, index: int = 0):
+    def delete_nth(self, index: int = 0) -> Any:
         if not 0 <= index < len(self):
             raise IndexError("list index out of range.")
         delete_node = self.head
@@ -76,7 +76,7 @@ class CircularLinkedList:
                 self.tail = temp
         return delete_node.data
 
-    def is_empty(self):
+    def is_empty(self) -> bool:
         return len(self) == 0
 
 

data_structures/linked_list/has_loop.py

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, Optional
 
 
 class ContainsLoopError(Exception):
@@ -7,8 +7,8 @@ class ContainsLoopError(Exception):
 
 class Node:
     def __init__(self, data: Any) -> None:
-        self.data = data
-        self.next_node = None
+        self.data: Any = data
+        self.next_node: Optional[Node] = None
 
     def __iter__(self):
         node = self

data_structures/linked_list/middle_element_of_linked_list.py

@@ -1,5 +1,8 @@
+from typing import Optional
+
+
 class Node:
-    def __init__(self, data: int) -> int:
+    def __init__(self, data: int) -> None:
         self.data = data
         self.next = None
 
@@ -14,7 +17,7 @@ class LinkedList:
         self.head = new_node
         return self.head.data
 
-    def middle_element(self) -> int:
+    def middle_element(self) -> Optional[int]:
         """
         >>> link = LinkedList()
         >>> link.middle_element()
@@ -54,6 +57,7 @@ class LinkedList:
             return slow_pointer.data
         else:
             print("No element found.")
+            return None
 
 
 if __name__ == "__main__":

data_structures/linked_list/skip_list.py

@@ -5,14 +5,14 @@ https://epaperpress.com/sortsearch/download/skiplist.pdf
 from __future__ import annotations
 
 from random import random
-from typing import Generic, TypeVar
+from typing import Generic, Optional, TypeVar, Union
 
 KT = TypeVar("KT")
 VT = TypeVar("VT")
 
 
 class Node(Generic[KT, VT]):
-    def __init__(self, key: KT, value: VT):
+    def __init__(self, key: Union[KT, str] = "root", value: Optional[VT] = None):
         self.key = key
         self.value = value
         self.forward: list[Node[KT, VT]] = []
@@ -49,7 +49,7 @@ class Node(Generic[KT, VT]):
 
 class SkipList(Generic[KT, VT]):
     def __init__(self, p: float = 0.5, max_level: int = 16):
-        self.head = Node("root", None)
+        self.head: Node[KT, VT] = Node[KT, VT]()
         self.level = 0
         self.p = p
         self.max_level = max_level