Check if Array Contains Duplicate Elements Using Hash Set

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

Explanation: Use a hash set to track seen elements. If an element repeats, return true.

⏱ O(n) | 💾 O(n)

class Solution {
  public boolean containsDuplicate(int[] nums) {
    Set<Integer> set = new HashSet<>();
    for (int num : nums) {
      if (!set.add(num)) return true;
    }
    return false;
  }
}
                  

Explanation: HashSet.add returns false if element exists. So duplicates can be detected immediately.

⏱ O(n) | 💾 O(n)

def contains_duplicate(nums):
    seen = set()
    for num in nums:
        if num in seen:
            return True
        seen.add(num)
    return False
                  

Explanation: A set tracks elements. Any repeat indicates a duplicate.

⏱ O(n) | 💾 O(n)