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6. Contains Duplicate

easyAsked at Postman

Return true if any value appears at least twice in an integer array.

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 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

Sort the array, then check adjacent pairs for equality.

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

Stream the array into a Set; if any value is already present, return true immediately.

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

Tradeoff:

Postman-specific tips

Postman uses set-based dedup heavily when canonicalizing header keys, so showing the Set short-circuit is the expected idiom.

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