/**
1. 优先级 乘除 > 加减 , 关键是怎么把 加减的计算放到乘除后面
2. 空格什么用都没有, 还得多加判断, 可以在之前全删掉
3. 可以用一个栈来保存:
    3.1 遍历到 + , - , 0~9 这些都入栈
    3.2 遍历到 乘除时 计算 栈顶与下一个值的结果压入栈中
    3.3 遍历结束后 栈中只有 + , - , 0~9  直接计算即可
*/

class Solution {
    public int calculate(String s) {
        Stack<String> stack = new Stack<>();
        s = s.replace(" ", "");
        char[] str = s.toCharArray();
        int n = str.length;

        for (int i = 0 ; i < n ; i++){
            if (str[i] == '+' || str[i] == '-'){
                stack.push(String.valueOf(str[i]));
            } else if (str[i] == '*' || str[i] == '/'){
                int x = Integer.parseInt(stack.pop());
                char sign = str[i];
                int y = 0;
                for (int j = i + 1; j < n && '0' <= str[j] && str[j] <= '9';i = j++) y = y * 10 + str[j] - '0';
                stack.push(String.valueOf(sign == '*' ? x * y : x / y));
            } else if ('0' <= str[i] && str[i] <= '9'){
                int y = 0;
                for (int j = i; j < n && '0' <= str[j] && str[j] <= '9';i = j++) num = num * 10 + str[j] - '0';
                stack.push(String.valueOf(y));
            }
        }

        int result = 0;
        while (stack.size() > 1){
            int x = Integer.parseInt(stack.pop());
            String sign = stack.pop();
            result += sign.equals("+") ?  x : 0 - x;
        }
        return result + Integer.parseInt(stack.pop());
    }
}