input: Write a function to evaluate a postfix expression and return the evaluated value. If the input to the program is 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 +")==5
  assert evaluate("5 4 -")==1
  assert evaluate("3 4 2 * +")==11
  assert evaluate("2 5 * 4 + 3 2 * 1 + *")==45
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]