input: Write a function to evaluate a postfix expression. The input to the function should be a string with the postfix expression. The output should be the evaluated value of the expression. If the input is an empty string then return 0.
  assert evaluate("2 3 + 4 5 * *")==46
  assert evaluate("3 4 + 5 *")==35
  assert evaluate("4 5 * 6 - 7 +")==11
  assert evaluate("")==0
  assert evaluate("10 20 +")==30
output: def evaluate(expression):
      if not expression:
          return 0
      stack = []
      tokens = expression.split()
      for token in tokens:
          if token.isdigit() or (token[0] == '-' and token[1:].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]