input: Write a python function to create a complex number class and return its string representation.
  assert str(ComplexNumber(1, 2)) == "1 + 2i"
  assert str(ComplexNumber(3, 4)) == "3 + 4i"
  assert str(ComplexNumber(-1, -1)) == "-1 + -1i"
output: class ComplexNumber:
      def __init__(self, real, imag):
          self.real = real
          self.imag = imag
  
      def __str__(self):
          return f"{self.real} + {self.imag}i"