input: Write a python function to determine the type of a triangle based on its side lengths.
  assert triangle_type(3, 3, 3) == 'Equilateral'
  assert triangle_type(3, 4, 4) == 'Isosceles'
  assert triangle_type(3, 4, 5) == 'Scalene'
  assert triangle_type(1, 2, 3) == 'Invalid'
output: def triangle_type(a, b, c):
      if a + b <= c or a + c <= b or b + c <= a:
          return 'Invalid'
      elif a == b == c:
          return 'Equilateral'
      elif a == b or b == c or a == c:
          return 'Isosceles'
      else:
          return 'Scalene'