使用栈的栈（准确地说是双向队列的双向队列）。记录一下
class Solution(object):
def calculate(self, s):
def add(x,y):
return x+y
def minus(x,y):
return x-y
outer_stack=[]
inner_stack=[]
num=None
for c in s:
if c.isdigit():
num = 0 if num is None else num
num = 10 * num + int(c)
continue
if c=='(':
if inner_stack!='':
outer_stack.append(inner_stack)
inner_stack=[]
continue
if c=='+':
if num is not None:
inner_stack.append(num)
num=None
inner_stack.append(add)
if len(inner_stack)>=3:
x=inner_stack.pop(0)
ope=inner_stack.pop(0)
y=inner_stack.pop(0)
inner_stack.insert(0,ope(x,y))
continue
if c=='-':
if num is not None:
inner_stack.append(num)
num=None
inner_stack.append(minus)
if len(inner_stack)>=3:
x=inner_stack.pop(0)
ope=inner_stack.pop(0)
y=inner_stack.pop(0)
inner_stack.insert(0,ope(x,y))
if c==')':
if num is not None:
inner_stack.append(num)
num=None
x=inner_stack.pop(0)
while len(inner_stack)>0:
ope=inner_stack.pop(0)
y=inner_stack.pop(0)
x=ope(x,y)
if len(outer_stack)>0:
inner_stack=outer_stack.pop()
inner_stack.append(x)
if num is not None:
inner_stack.append(num)
while len(inner_stack)>0 or len(outer_stack)>0:
x=inner_stack.pop(0)
while len(inner_stack)>0:
ope=inner_stack.pop(0)
y=inner_stack.pop(0)
x=ope(x,y)
if len(outer_stack)>0:
inner_stack=outer_stack.pop()
inner_stack.append(x)
return x
