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10. Contains Duplicate

easyAsked at Coupang

Detect whether any value appears twice in an array, mirroring how Coupang's returns-processing pipeline deduplicates RMA submissions.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an integer array nums, return true if any value appears at least twice and 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

Example 2

Input
nums=[1,2,3,4]
Output
false

Approaches

1. Brute force

Check every pair.

Time
O(n^2)
Space
O(1)
for (let i = 0; i < nums.length; i++)
  for (let j = i + 1; j < nums.length; j++)
    if (nums[i] === nums[j]) return true;
return false;

Tradeoff:

2. Hash set

Insert each value into a set; if it's already present, you've found a duplicate.

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:

Coupang-specific tips

Coupang returns-processing rejects duplicate RMA submissions; the Set-based O(n) sweep is the standard pattern in their backend codebase.

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