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Contains Duplicate

Return true if any value appears at least twice in the array; otherwise false.

Answers use simple, clear English.

Audio N/A

Time: O(n)Space: O(n)

Quick interview answer

Insert into a hash set while iterating. If an element is already present, return true. Sorting also works in O(n log n) with O(1) extra if mutation is allowed.

Detailed answer

Insert into a hash set while iterating. If an element is already present, return true. Sorting also works in O(n log n) with O(1) extra if mutation is allowed. Set approach is the default interview answer unless memory is constrained.

Full explanation

Set approach is the default interview answer unless memory is constrained.

Real example & use case

Deduplicating uploaded file hashes to reject repeat uploads in a media pipeline.

Pros & cons

Pros: expected O(n). Cons: O(n) memory; sorting trades time for space.

Code example

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

Practice code · python (view only · no execution)

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

#hashset#array