input: Write a function to find all the divisors of a given number and print them. If the number is negative, return an error message.
  assert find_divisors(12)==[1, 2, 3, 4, 6, 12]
  assert find_divisors(7)==[1, 7]
  assert find_divisors(-5)=='Number must be non-negative'
  assert find_divisors(0)==[0]
output: def find_divisors(n):
      if n < 0:
          return 'Number must be non-negative'
      divisors = []
      for i in range(1, n + 1):
          if n % i == 0:
              divisors.append(i)
      return divisors