217. Contains Duplicate

At a Glance

• Topic: Array
• Pattern: Hash Set
• Difficulty: Easy
• LeetCode: 217

Problem Statement

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.

Example 1:

Input: nums = [1,2,3,1]

Output: true

Explanation:

The element 1 occurs at the indices 0 and 3.

Example 2:

Input: nums = [1,2,3,4]

Output: false

Explanation:

All elements are distinct.

Example 3:

Input: nums = [1,1,1,3,3,4,3,2,4,2]

Output: true

Constraints:

1 <= nums.length <= 105
-109 <= nums[i] <= 109

Approach & Solution Steps

Use a hash set to store seen numbers. Iterate through the array; if a number is already in the set, return true. Otherwise, add it and continue.

Optimal JS Solution

function containsDuplicate(nums) {
  const set = new Set();
  for (const num of nums) {
    if (set.has(num)) return true;
    set.add(num);
  }
  return false;
}

Edge Cases & Pitfalls

• Always consider empty or null inputs.
• Watch out for off-by-one index errors.