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Optimize Node.has_loop to use constant auxiliary space #15607

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Optimize Node.has_loop to use constant auxiliary space #15607

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29 changes: 24 additions & 5 deletions data_structures/linked_list/has_loop.py
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Original file line number Diff line number Diff line change
Expand Up @@ -36,12 +36,28 @@ def has_loop(self) -> bool:
>>> root_node.next_node.next_node.next_node = root_node.next_node
>>> root_node.has_loop
True
>>> single_node = Node(1)
>>> single_node.next_node = single_node
>>> single_node.has_loop
True
>>> loop_to_root = Node(1)
>>> loop_to_root.next_node = Node(2)
>>> loop_to_root.next_node.next_node = loop_to_root
>>> loop_to_root.has_loop
True
>>> dup_node = Node(5)
>>> dup_node.next_node = Node(5)
>>> dup_node.has_loop
False
"""
try:
list(self)
return False
except ContainsLoopError:
return True
slow: Node | None = self
fast: Node | None = self
while fast and fast.next_node and slow:
slow = slow.next_node
fast = fast.next_node.next_node
if slow is fast:
return True
return False


if __name__ == "__main__":
Expand All @@ -61,3 +77,6 @@ def has_loop(self) -> bool:

root_node = Node(1)
print(root_node.has_loop) # False

root_node.next_node = root_node
print(root_node.has_loop) # True
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