input: Write a python function to evaluate a postfix expression.
  assert evaluate("2 3 + 4 5 * *") == 46
  assert evaluate("5 1 2 + 4 * + 3 -") == 14
  assert evaluate("2 3 1 * + 9 -") == -4
output: def evaluate(expression: str) -> int:
      stack = []
      operators = {'+', '-', '*', '/'}
      
      for token in expression.split():
          if token not in operators:
              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(int(a / b))  # use int() for floor division
              
      return stack[0]