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23. Contains Duplicate

easyAsked at Brex

Detect whether any value appears at least twice in an array — a hash set warm-up that Brex uses to establish baseline data structure fluency.

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

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

Example 2

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

Approaches

1. Sort and compare adjacent

Sort the array, then check if any consecutive pair is equal.

Time
O(n log n)
Space
O(1)
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 is already present return true immediately. Single pass O(n) time and 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:

Brex-specific tips

Brex asks about fintech infrastructure, multi-currency handling, and spend management algorithms. Expect LeetCode-style DSA focused on hash maps, sorting, and dynamic programming.

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Output

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