input: Write a function to check if a number is a power of two.
  assert is_power_of_two(32) == True
  assert is_power_of_two(0) == False
  assert is_power_of_two(1) == True
  assert is_power_of_two(18) == False
  assert is_power_of_two(64) == True
output: def is_power_of_two(n):
      if n <= 0:
          return False
      return (n & (n - 1)) == 0