224

class Solution {
public:
    int calculate(string s) {
        long long int result=0;
        long long int num=0, sign=1;
        stack<long long int> nums, ops;

        for(int i=0; i<s.size(); ++i){
            char c=s[i];
            if(c>='0' && c<='9'){
                num=10*num + c-'0'; /// For case: "23"
            }else if(c=='+'){
                result += num*sign; /// everytime meets an operator, sum up previous number
                num=0;
                sign=1;    /// sign is the sign of next number
            }else if(c=='-'){
                result += num*sign; /// everytime meets an operator, sum up previous number
                num=0;
                sign=-1;
            }else if(c=='('){
                nums.push(result); /// ...(1+3+(..xx..)+...)... before go into cur (..xx..), record the forefront result (in this case it is 1+3 ) into nums array
                ops.push(sign);  // record cur (..xx..) sign
                result=0;  // result is to record the total value in the cur (..xx..)
                sign=1;
            }else if(c==')' && ops.size()){
                result += num*sign; /// For case: 1-(5)
                num=0;
                result = result*ops.top() + nums.top();  // ...(1+3+(..xx..)+...)... sum up cur (..xx..)  and the forefront result (in this case it is 1+3 )
                nums.pop();
                ops.pop();
            }
        }
        result += num*sign; /// For the last one operation. consider the case:  1+2+3
        return result;
    }
};

-----------------------------

class Solution {
public:
    int calculate(string s) {
        int n = s.size(), ret = 0, sign = 1;
        stack<int> stk;

        for (int i = 0; i < n; i++) {
            if (isdigit(s[i])) {
                int num = s[i] - '0';

                while (i + 1 < n && isdigit(s[i + 1])) {
                    num = num * 10 + (s[i + 1] - '0');
                    i++;
                }

                ret += sign * num;
            } else if (s[i] == '+') {
                sign = 1;
            } else if (s[i] == '-') {
                sign = -1;
            } else if (s[i] == '(') {
                stk.push(ret);
                stk.push(sign);
                ret = 0;
                sign = 1;
            } else if (s[i] == ')') {
                ret *= stk.top();
                stk.pop();
                ret += stk.top();
                stk.pop();
            }
        }

        return ret;
    }
};