Floyd · detect and locate

A slow and a fast pointer meet if a linked list cycles. Reset one to the head and move both one step to locate the entrance.

Remember: Meet first; reset one; walk together.

Example: 10 → 20 → 30 → 20 has cycle entrance 20.

Time: O(n). Auxiliary space: O(1).

Formula: slow moves 1; fast moves 2

Tail links to the selected zero-based node index; −1 means no cycle. Collision is not necessarily the entrance.

Complexities describe the underlying algorithm; tutorial snapshots and rendering can use additional resources.

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