224. Basic Calculator

class Solution {
public:
    int calculate(string s) {
        int res = 0, sign = 1, x = 0;
        stack<int> nums, ops;
        for (char c:s) {
            if (isdigit(c)) x = x * 10 + c - '0';
            else {
                res += sign * x; x = 0; 
                if (c == '-' || c == '+') sign = (c == '+') ? 1 : -1;
                if (c == '(') {
                    nums.push(res); ops.push(sign);
                    res = 0; sign = 1;
                }
                if (c == ')') {
                    res = nums.top() + ops.top() * res;
                    nums.pop(); ops.pop();
                }
            }
        }
        return res + sign * x;
    }
};