Contains Duplicate

  # 🔁 2. Contains Duplicate
  # Returns true if the input array has any duplicates, false otherwise.
  # Uses a Set to keep track of seen elements.
  # Time Complexity: O(n)
  # Why: Single pass through the array while checking and adding to a Set.
  def contains_duplicate(nums)
    seen = Set.new
    nums.each do |num|
      return true if seen.include?(num)
      seen.add(num)
    end
    false