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.
  assert evaluate("2 3 4 * +")==14
  assert evaluate("2 3 + 4 *")==20
  assert evaluate("7 8 + 3 *")==45
  assert evaluate("5 3 2 * +")==11
  assert evaluate("10 2 3 ^ -")==-1
output: def evaluate(expression):
      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)
              elif token == '^':
                  stack.append(a ** b)
      return stack[0]