public class DayCode {    public static void main(String[] args) {        String s = "1+2";        int ans = new DayCode().calculate(s);        System.out.println("ans is " + ans);    }
    /**     * https://leetcode-cn.com/problems/basic-calculator/     * 时间复杂度O(n)     * 空间复杂度O(n)     * @param s     * @return     */    private int idx = 0;
    public int calculate(String s) {        Deque<Integer> stack = new ArrayDeque<>();        s = s.trim();        char opt = '+';        int num = 0;        while (idx < s.length()) {            char c = s.charAt(idx);            if (c == '(') {                idx++;                num = calculate(s);                if (idx < s.length()) {                    c = s.charAt(idx);                }            }
            if (c == ' ') {                idx++;                continue;            }            if (c >= '0' && c <= '9') {                num = num * 10 + (c - '0');            }            if (!(c >= '0' && c <= '9') || (idx == s.length() - 1)) {                switch (opt) {                    case '+':                        stack.push(num);                        break;                    case '-':                        stack.push(-num);                        break;                    default:                        break;                }                num = 0;                opt = c;            }            idx++;            if (c == ')') {                break;            }        }
        int ans = 0;        while (!stack.isEmpty()) {            ans += stack.pop();        }        return ans;    }
}