150. 逆波兰表达式求值

1. [题目链接](150. 逆波兰表达式求值 - 力扣（LeetCode）)

2. 解题思路：用一个栈，遇到数字压进去，遇到运算符则计算后压入栈

3. 代码

   class Solution:
       def evalRPN(self, tokens: List[str]) -> int:
           stack = []
           ans = 0
           for s in tokens:
               if s == '+':
                   a = stack.pop()
                   b = stack.pop()
                   stack.append(a + b)
               elif s == '-':
                   a = stack.pop()
                   b = stack.pop()
                   stack.append(b - a)
               elif s == '*':
                   a = stack.pop()
                   b = stack.pop()
                   stack.append(a * b)
               elif s == "/":
                   a = stack.pop()
                   b = stack.pop()
                   stack.append(math.trunc(b / a))
               else:
                   stack.append(int(s))
           return stack.pop()