# Leetcode 227. Basic Calculator II

## 題解

### Stack solution

先把 **\*** 和 **/** 處理完，再把所有的數字加起來，遇到 **-** 轉換數字為負數

class Solution:
    def calculate(self, strings: str) -> int:
        n = len(strings)
        if n == 0 or not strings: return 0
        stack = []
        cur_num = 0
        op = "+"
        for i in range(n):
            s = strings[i]
            if s.isdigit():
                cur_num = (cur_num * 10) + int(s)
            if not s.isdigit() and s != " " or i == n - 1: # 處理到最後一個的時候，需要把最後的 operator 順便一起處理掉
                if op == "*":
                    stack.append(stack.pop() * cur_num)
                elif op == "/":
                    stack.append(int(stack.pop() / cur_num))
                elif op == "+":
                    stack.append(cur_num)
                elif op == "-":
                    stack.append(cur_num * -1)
                op = s
                cur_num = 0
        return sum(stack)