What is the O(n) way to find duplicate in array?

Uses Floyd's cycle detection on array-as-graph. Assumes n+1 elements valued 1 to n. Tortoise-hare pointers find cycle entry: the dupe.

Does this work on any array?

Nope. Strict constraints: exactly one dupe, integers 1-n in n+1 spots. General case

No extra structures — just indices. Hash needs n buckets worst-case.

💻 Programming Languages

Hash sets and sorts? So 2023. This O(n) duplicate finder uses cycle detection magic. But only if your array plays by strict rules.

theAIcatchup Apr 11, 2026 3 min read

O(n) time and O(1) space duplicate finding relies on pigeonhole constraints: n+1 elements from 1 to n. 𝕏
It's Floyd's cycle detection repurposed — clever graph thinking, not new magic. 𝕏
Great for interviews, niche for production; don't overhype beyond the rules. 𝕏

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#algorithms #arrays #cycle detection #time complexity

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Originally reported by Reddit r/programming