added linked lists

leetcode/linked_list_cycle.py

@@ -0,0 +1,26 @@
+# Given head, the head of a linked list, determine if the linked list has a cycle in it.
+
+# There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. 
+# Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.
+
+# Return true if there is a cycle in the linked list. Otherwise, return false.
+
+# https://leetcode.com/problems/linked-list-cycle/description/
+
+from typing import Optional
+
+class ListNode:
+    def __init__(self, x):
+        self.val = x
+        self.next = None
+
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        fast = slow = head
+
+        while fast and fast.next:
+            slow, fast = slow.next, fast.next.next
+            if fast == slow:
+                return True
+
+        return False

stepik/linked_list/linked_list.py

@@ -11,26 +11,47 @@ def __init__(self):
         self.top = Node()
         self.tail = None
 
+    def append_to_start(self, value):
+        """
+        Добавление нового элемента в начало связного списка.
+        Время работы O(1).
+        """
+
+        new_node = Node(value)
+        new_node.next_node = self.top.next_node
+        self.top.next_node = new_node
+        if self.tail is None:
+            self.tail = new_node
+
     def append_to_end(self, value):
         """
         Добавление нового элемента в конец связного списка.
         Время работы O(1).
         """
 
-        tail = self.tail
-        if tail is None:
-            tail = Node()
-
-        tail.next_node = Node(value)
+        new_node = Node(value)
 
-    def append_to_start(self, value):
+        if self.tail is not None:
+            self.tail.next_node = new_node
+            self.tail = new_node
+            return
+
+        self.top.next_node = new_node
+        self.tail = new_node
+
+    def append_after_target(self, target, value):
         """
-        Добавление нового элемента в начало связного списка.
-        Время работы O(1).
+        Добавление элемента после некоторого значения.
         """
 
-        node = Node(value=value, next_node=self.top.next_node)
-        self.top.next_node = node
+        current = self.top.next_node
+        node = Node(value)
+
+        while current is not None:
+            if current.value == target:
+                node.next_node = current.next_node
+                current.next_node = node
+            current = current.next_node
 
     def delete(self, value):
         """
@@ -71,5 +92,5 @@ def __str__(self):
 l.append_to_start(1)
 l.append_to_end(2)
 l.append_to_end(3)
-
+l.append_after_target(1, 4)
 print(l)