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15. Contains Duplicate

easyAsked at Figma

Determine whether an integer array contains any duplicates. Figma frames this as a sanity gate before moving to scene-graph dedup problems.

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 scan adjacent

Sort the array and compare adjacent pairs for equality.

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 early exit

Insert into a Set; the first collision proves a duplicate exists. Single pass, early exit beats the sort approach in expected case.

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

Tradeoff:

Figma-specific tips

Figma watches whether you mention the O(n) vs O(n log n) trade and pick Set for expected-case wins — they care about pragmatic constant factors on canvas-sized inputs.

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