input: 
  def count_pairs(numbers: List[int], x: int) -> int:
      """ Count the number of pairs in the list which sum to x.
      >>> count_pairs([1, 2, 3, 4, 5, 6], 7)
      3
      >>> count_pairs([2, 7, 11, 15], 9)
      1
      """
      count = 0
      i, j = 0, len(numbers) - 1
      while i < j:
          sum = numbers[i] + numbers[j]
          if sum == x:
              count += 1
              i += 1
          elif sum < x:
              i += 1
          else:
              j -= 1
      return count
  
output:     count = 0
      seen = set()
      for num in numbers:
          complement = x - num
          if complement in seen:
              count += 1
          seen.add(num)
      return count