# Leetcode 150. Evaluate Reverse Polish Notation

## 題解

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        eval_chr = {'+','-','*','/'}
        for s in tokens:
            if s in eval_chr:
                n2 = stack[-1]
                stack.pop()
                n1 = stack[-1] # 後面的先
                stack.pop()
                ans = 0
                if s == "+":
                    ans = str(int(n1) + int(n2))
                elif s == "-":
                    ans = str(int(n1) - int(n2))
                elif s == "*":
                    ans = str(int(n1) * int(n2))
                elif s == "/":
                    ans = str(int(int(n1) / int(n2)))
                stack.append(ans)
            else:
                stack.append(s)
        return int(stack[0])