input: Write a function to evaluate a postfix expression. The function should take a string as input with the postfix expression and return the evaluated value of the expression. If the input string is empty then return 0.
  assert evaluate("2 3 + 4 5 * *")==46
  assert evaluate("5 6 + 2 3 * +")==17
  assert evaluate("3 4 + 5 *")==35
  assert evaluate("")==0
  assert evaluate("10 2 8 * + 3 -")==23
output: def evaluate(expression):
      if not expression:
          return 0
      stack = []
      tokens = expression.split()
      for token in tokens:
          if token.isdigit():
              stack.append(int(token))
          else:
              b = stack.pop()
              a = stack.pop()
              if token == '+':
                  stack.append(a + b)
              elif token == '-':
                  stack.append(a - b)
              elif token == '*':
                  stack.append(a * b)
              elif token == '/':
                  stack.append(a / b)
      return stack[0]