input: Write a python function to determine if a given number can be expressed as the sum of two perfect squares.
  assert sum_of_squares(25) == True
  assert sum_of_squares(24) == False
  assert sum_of_squares(17) == True
output: def sum_of_squares(n):
      for i in range(int(n**0.5)+1):
          j = n - i*i
          if j >= 0 and int(j**0.5)**2 == j:
              return True
      return False