leetcode 150. 逆波兰表达式求值
150. 逆波兰表达式求值 - 力扣（LeetCode）
class Solution:
def evalRPN(self, tokens: List[str]) -> int:
stack=[]
for item in tokens:
if item not in ( '+','-','*' , '/' ):
stack.append(item)
else:
pre=int(stack.pop())
pre_before=int(stack.pop())
sign =item
if sign =='/':
result=int(pre_before/pre)
elif sign =='+':
result=pre_before+pre
elif sign =='-':
result=pre_before-pre
elif sign =='*':
result=pre_before*pre
stack.append(result)
return int(stack.pop())