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How do you detect a cycle in a linked list?

Q: How do you detect a cycle in a linked list?

**Intuition.** Use **Floyd's Tortoise and Hare**: two pointers, one stepping by 1 and one by 2. If the list ends, there is no cycle. If there *is* a loop, the fast pointer laps the slow one and they collide — like two runners on a circular track. O(1) extra space, no hash set needed. 1 2 <text x="220" y="6

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Floyd's cycle detection

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class ListNode:
    def __init__(self, val, nxt=None):
        self.val, self.next = val, nxt

def has_cycle(head):
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow is fast:
            return True
    return False


# --- demo ---
a, b, c, d = ListNode(1), ListNode(2), ListNode(3), ListNode(4)
a.next, b.next, c.next, d.next = b, c, d, b   # tail links back -> cycle
print(has_cycle(a))                            # True
x = ListNode(1); x.next = ListNode(2)
print(has_cycle(x))                            # False

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