任意输入含加减乘除乘方开方的文本数学表达式，使用二叉树数据结构，使用堆栈算法，生成计算结果

def calculate(string):
  operator = {
    '+': (1, lambda a, b: a + b), 
    '-': (1, lambda a, b: a - b), 
    '*': (2, lambda a, b: a * b), 
    '/': (2, lambda a, b: a / b), 
    '//': (2, lambda a, b: a // b), 
    '%': (2, lambda a, b: a % b), 
    '**': (3, lambda a, b: a ** b)
  }
  stack = []
  for item in string.split(' '):
    if item in operator:
      while stack and operator[item][0] <= operator[stack[-1]][0]:
        a, b = stack.pop(), stack.pop()
        stack.append(operator[b][1](a, b))
      stack.append(item)
    elif item:
      stack.append(float(item))
  while len(stack) != 1:
    a