input: Write a function to construct a polynomial from a string and evaluate it.
  assert construct_polynomial("3+2+1")(2) == 17
  assert construct_polynomial("1+1+1")(3) == 13
  assert construct_polynomial("5+0+3")(1) == 8
output: def construct_polynomial(poly_str):
      def evaluate(x):
          terms = list(map(int, poly_str.split('+')))
          return sum(coef * (x ** exp) for exp, coef in enumerate(reversed(terms)))
      return evaluate