public class DayCode {    public static void main(String[] args) {        String s = "1 + 2 * 3";        int ans = new DayCode().calculate(s);        System.out.println("ans is " + ans);    }
    /**     * https://leetcode-cn.com/problems/basic-calculator-ii/     * 时间复杂度O(n)     * 空间复杂度O(n)     * @param s     * @return     */    public int calculate(String s) {        Deque<Integer> stack = new ArrayDeque<>();        char preSign = '+';        int num = 0;        int n = s.length();        for (int i = 0; i < n; i++) {            if (Character.isDigit(s.charAt(i))) {                num = num * 10 + s.charAt(i) - '0';            }            if (!Character.isDigit(s.charAt(i)) && s.charAt(i) != ' ' || i == n - 1) {                switch (preSign) {                    case '+':                        stack.push(num);                        break;                    case '-':                        stack.push(-num);                        break;                    case '*':                        stack.push(stack.pop() * num);                        break;                    case '/':                        stack.push(stack.pop() / num);                        break;                    default:                        break;                }                preSign = s.charAt(i);                num = 0;            }        }
        int ans = 0;        while (!stack.isEmpty()) {            ans += stack.pop();        }        return ans;    }}