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Question 39 of 100

Part 5: Linked Lists & Fast-Slow Pointers (37 - 45)

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DSA Patterns (Multi-Lang)•Part 5: Linked Lists & Fast-Slow Pointers (37 - 45)

Linked List Cycle (Floyd’s Tortoise and Hare).

In Simple Words: Slow pointer moves 1 step, fast pointer moves 2 steps. If there is a cycle, fast will eventually lap slow and slow === fast. If fast hits null, no cycle exists.

  • Time: $O(N)$ | Space: $O(1)$
Multi-Language Solutions
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def hasCycle(head: ListNode | None) -> bool:
    slow, fast = head, head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast:
            return True
    return False
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