Contains Duplicate

easyTests passed 2026-11-11v1.0.0

TC: O(N)|SC: O(N)

Arrays & Hashing Original Statement (leetcode).md

1. Core Invariant / The Trick

Maintain a hash set of previously seen numbers. Return True immediately if the current element is already in the set; otherwise insert and continue.

2. Complexity Bounds & Proof

Single linear pass inserting at most N distinct elements with O(1) expected set operations.

3. Minimal Idiomatic Code

def containsDuplicate(nums: list[int]) -> bool:
    seen = set()
    for x in nums:
        if x in seen:
            return True
        seen.add(x)
    return False

4. Failure Modes & Interview Follow-ups

Pitfalls:- Sorting mutates the input array and takes O(N log N) time; prefer O(N) hash set unless constrained by strict O(1) auxiliary space.

Follow-ups:- Contains Duplicate II (within distance k): maintain a sliding window set of size <= k.

Personal Notes & Scratchpad (Local-First Yjs CRDT)

Saved instantaneously in IndexedDB. Streams live across peers via Hocuspocus when signed in.

Related Invariants

Two Sum (O(N))Valid Anagram (O(N))

Browser Scratchpad

Console Output

Click Run or press Cmd/Ctrl+Enter to execute locally in a Web Worker.