input: Write a function to evaluate a postfix expression given as a string. The function should return the evaluated value of the expression. If the expression is empty, return 0.
  def evaluate(expression):
      if expression == "":
          return 0
      expression = expression.split()
      stack = []
      for element in expression:
          if element.isdigit():
              stack.append(element)
          else:
              val1 = stack.pop()
              val2 = stack.pop()
              stack.append(str(eval(val2 + element + val1)))
      return (int(stack[0]))
  assert evaluate("2 3 + 4 5 * *") == 46
  assert evaluate("10 2 8 * + 3 -") == 23
  assert evaluate("5 1 2 + 4 * + 3 -") == 14
  assert evaluate("") == 0
  assert evaluate("2 3 +") == 5
output: def evaluate(expression):
      if expression == "":
          return 0
      expression = expression.split()
      stack = []
      for element in expression:
          if element.isdigit():
              stack.append(int(element))
          else:
              val1 = stack.pop()
              val2 = stack.pop()
              if element == '+':
                  stack.append(val2 + val1)
              elif element == '-':
                  stack.append(val2 - val1)
              elif element == '*':
                  stack.append(val2 * val1)
              elif element == '/':
                  stack.append(val2 / val1)
      return stack[0]