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8. Contains Duplicate

easyAsked at Klarna

Detect whether an array contains any duplicate value.

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.

Time
O(n^2)
Space
O(1)
function containsDuplicate(nums) {
  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 element into a set; if we see a member already present, return true. Early-exit keeps it fast in practice.

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:

Klarna-specific tips

Klarna uses this exact pattern to dedupe merchant-settlement events; expect a follow-up about how you'd scale this to a 10B-row Kafka stream when you can't fit the set in memory.

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