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Problem

Linked List Cycle

LeetCode #141Easy
Floyd's tortoise & hare

Given the head of a singly linked list, determine whether the list contains a cycle, meaning some node can be reached again by repeatedly following next pointers.

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step 1 / 14
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Brute force · visited set
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1given head
2visited = {}
3cur = head; while cur ≠ ∅:
4    if cur in visited: return true
5    add cur; cur = cur.next
6return false
state
  • cycle??

line 1Does this list loop forever? The red arrow makes node 3 point BACK to node 1 — a walker never reaches ∅. Obvious fix: remember every node you visit.