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13. Contains Duplicate

easyAsked at N26

Return true if any value appears at least twice in the array. N26 uses this as the simplest version of their idempotency-key duplicate-detection check.

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

Compare every pair of indices.

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 early exit

Insert into a set; if the value is already present you have your duplicate. Constant-time lookup keeps the scan linear.

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:

N26-specific tips

N26 likes you to map this exact pattern to their idempotency-key store: a duplicate POST to the SEPA initiation endpoint must short-circuit before a second debit fires.

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Output

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