16. Contains Duplicate

easyAsked at Unity

Detect repeated values in an array — the same deduplication check Unity's asset import pipeline runs when the editor scans a folder for duplicate mesh GUIDs before loading a scene.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-26

Problem

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

Constraints

1 <= nums.length <= 10^5
-10^9 <= nums[i] <= 10^9

Examples

Example 1

Input

nums = [1,2,3,1]

Output

true

Explanation: 1 appears at indices 0 and 3.

Example 2

Input

nums = [1,2,3,4]

Output

false

Approaches

1. Sorting

Sort the array; duplicates become adjacent. One scan suffices.

Time: O(n log n)
Space: O(1)

function containsDuplicate(nums) {
  nums.sort((a, b) => a - b);
  for (let i = 1; i < nums.length; i++) {
    if (nums[i] === nums[i - 1]) return true;
  }
  return false;
}

Tradeoff:

2. Hash set single pass

Insert each element into a Set. If it already exists, a duplicate is found. O(n) time, O(n) space.

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

function containsDuplicate(nums) {
  const seen = new Set();
  for (const n of nums) {
    if (seen.has(n)) return true;
    seen.add(n);
  }
  return false;
}

Tradeoff:

Unity-specific tips

Unity interviewers tie this directly to asset deduplication — articulate the Set as a GUID registry that the import pipeline checks on every file discovery event, and mention the memory cost is acceptable given scene sizes are bounded.

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Output

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