Add cycle detection and removal in linked list

data_structures/linked_list/cycle_detection_and_removal.py

@@ -0,0 +1,120 @@
+from __future__ import annotations
+
+from typing import Any
+import doctest
+
+
+class ContainsLoopError(Exception):
+    pass
+
+
+class Node:
+    def __init__(self, data: Any) -> None:
+        self.data: Any = data
+        self.next_node: Node | None = None
+
+    def __iter__(self):
+        node = self
+        visited = []
+        while node:
+            if node in visited:
+                raise ContainsLoopError
+            visited.append(node)
+            yield node.data
+            node = node.next_node
+
+    @property
+    def has_loop(self) -> bool:
+        """
+        A loop is when the exact same Node appears more than once in a linked list.
+        >>> root_node = Node(1)
+        >>> root_node.next_node = Node(2)
+        >>> root_node.next_node.next_node = Node(3)
+        >>> root_node.next_node.next_node.next_node = Node(4)
+        >>> root_node.has_loop
+        False
+        >>> root_node.next_node.next_node.next_node = root_node.next_node
+        >>> root_node.has_loop
+        True
+        """
+        try:
+            list(self)
+            return False
+        except ContainsLoopError:
+            return True
+
+    def remove_loop(self) -> None:
+        """
+        Removes the loop in the linked list if one exists.
+        Wikipedia Reference: [Linked list](https://en.wikipedia.org/wiki/Linked_list)
+
+        >>> root_node = Node(1)
+        >>> root_node.next_node = Node(2)
+        >>> root_node.next_node.next_node = Node(3)
+        >>> root_node.next_node.next_node.next_node = Node(4)
+        >>> root_node.remove_loop()
+        >>> root_node.has_loop
+        False
+
+        >>> root_node = Node(1)
+        >>> root_node.next_node = Node(2)
+        >>> root_node.next_node.next_node = Node(3)
+        >>> root_node.next_node.next_node.next_node = Node(4)
+        >>> root_node.next_node.next_node.next_node = root_node.next_node
+        >>> root_node.remove_loop()
+        >>> root_node.has_loop
+        False
+
+        >>> root_node = Node(5)
+        >>> root_node.next_node = Node(6)
+        >>> root_node.next_node.next_node = Node(5)
+        >>> root_node.next_node.next_node.next_node = Node(6)
+        >>> root_node.remove_loop()
+        >>> root_node.has_loop
+        False
+        """
+        if self.has_loop:
+            slow_ptr = self
+            fast_ptr = self
+            while slow_ptr and fast_ptr and fast_ptr.next_node:
+                slow_ptr = slow_ptr.next_node
+                fast_ptr = fast_ptr.next_node.next_node
+                if slow_ptr == fast_ptr:
+                    break
+
+            # Move one pointer to the head of the linked list
+            slow_ptr = self
+            while slow_ptr.next_node != fast_ptr.next_node:
+                slow_ptr = slow_ptr.next_node
+                fast_ptr = fast_ptr.next_node
+
+            # Remove the loop
+            fast_ptr.next_node = None
+
+
+if __name__ == "__main__":
+    doctest.testmod()
+    root_node = Node(1)
+    root_node.next_node = Node(2)
+    root_node.next_node.next_node = Node(3)
+    root_node.next_node.next_node.next_node = Node(4)
+    print(root_node.has_loop)  # False
+    root_node.next_node.next_node.next_node = root_node.next_node
+    print(root_node.has_loop)  # True
+
+    # Remove the loop
+    root_node.remove_loop()
+    print(root_node.has_loop)  # False
+
+    root_node = Node(5)
+    root_node.next_node = Node(6)
+    root_node.next_node.next_node = Node(5)
+    root_node.next_node.next_node.next_node = Node(6)
+    print(root_node.has_loop)  # False
+
+    # Remove the loop
+    root_node.remove_loop()
+    print(root_node.has_loop)  # False
+
+    root_node = Node(1)
+    print(root_node.has_loop)  # False