input: Write a function to evaluate a postfix expression. The input to the program should be a string with the postfix expression. The output should be the evaluated value of the expression.
  assert evaluate("2 3 + 4 5 * *")==46
  assert evaluate("2 3 * 4 +")==10
  assert evaluate("10 2 - 5 *")==40
  assert evaluate("3 2 5 + -")==-4
  assert evaluate("6 4 2 / *")==12
output: def evaluate(expression):
      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]