input: Write a function to find pairs of numbers that add up to a given number.
  assert find_sum(5, [1, 2, 3, 4, 5]) == [[1, 4], [2, 3]]
  assert find_sum(10, [2, 3, 7, 8, 1, 9]) == [[2, 8], [3, 7], [1, 9]]
  assert find_sum(6, [1, 2, 3, 4, 5]) == [[1, 5], [2, 4]]
output: def find_sum(target, numbers):
      pairs = []
      seen = set()
      for num in numbers:
          complement = target - num
          if complement in seen:
              pairs.append([complement, num])
          seen.add(num)
      return pairs