Optimize Node.has_loop to use constant auxiliary space (Fixes #15576)

data_structures/linked_list/has_loop.py

@@ -1,63 +1,19 @@
-from __future__ import annotations
-
-from collections.abc import Iterator
-from typing import Any
-
-
-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) -> Iterator[Any]:
-        node = self
-        visited = set()
-        while node:
-            if node in visited:
-                raise ContainsLoopError
-            visited.add(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:
+@property
+def has_loop(self) -> bool:
+    """
+    Detect whether the linked list contains a cycle using Floyd's
+    Cycle-Finding Algorithm (Tortoise and Hare).
+
+    Time Complexity: O(n)
+    Auxiliary Space: O(1)
+    """
+    slow = self
+    fast = self.next
+
+    while fast is not None and fast.next is not None:
+        if slow is fast:
             return True
+        slow = slow.next
+        fast = fast.next.next
 
-
-if __name__ == "__main__":
-    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
-
-    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
-
-    root_node = Node(1)
-    print(root_node.has_loop)  # False
+    return False