input: Write a function to find the two largest numbers in an array.
  assert find_largest_two([3, 5, 2, 9, 1]) == (9, 5)
  assert find_largest_two([10, 20, 30, 40, 50]) == (50, 40)
  assert find_largest_two([5, 3, 8, 7, 2]) == (8, 7)
output: def find_largest_two(arr):
      if len(arr) < 2:
          return None
      first, second = float('-inf'), float('-inf')
      for num in arr:
          if num > first:
              second = first
              first = num
          elif num > second:
              second = num
      return first, second