input: 
  def evaluate(expression: str) -> int:
      """Evaluate a postfix expression.
      >>> evaluate("2 3 + 4 5 * *")
      46
      >>> evaluate("5 3 4 * +")
      17
      """
  
output:     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]