easy

Linked List Cycle

Given the head of a singly linked list, determine whether following next pointers ever revisits a node (i.e. the list contains a cycle). Use constant extra memory.

Constraints

0 ≤ number of nodes ≤ 10^4
O(1) space required

Examples

in: head = 3 → 2 → 0 → -4 → (back to 2)

out: true

in: head = 1 → 2

out: false

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