LeetCode: Basic Calculator

class Solution {
public:
    const int inf = 1e9;

    int Num(char a){return a - '0';}

    bool isNum(char a){return '0' <= a and a <= '9';}

    int calculate(string s) {
        int sz = s.size();
        stack <int> st;
        int sign = 1;
        for(int i=0;i<sz;i++){
            if(s[i] == ' ') continue;
            else if(s[i] == '+')
                sign = 1;
            else if(s[i] == '-')
                sign = -1;
            else if(isNum(s[i])){
                int num = 0;
                for(;i < sz and isNum(s[i]); i++){
                    num = 10 * num + Num(s[i]);
                }
                i --;
                st.push(sign);
                st.push(num);
            }
            else if(s[i] == '('){
                st.push(sign);
                st.push(inf);
                sign = 1;
            }
            else if(s[i] == ')'){
                int val = 0;
                int num = 0;
                int sign = 1;
                while(true){
                    num = st.top(); st.pop();
                    sign = st.top(); st.pop();
                    if(num == inf) break;
                    val += num * sign;
                }
                st.push(sign);
                st.push(val);
                sign = 1;
            }
        }
        int ans = 0;
        while(!st.empty()){
            int num = st.top(); st.pop();
            int sign = st.top(); st.pop();
            ans += num * sign;
        }
        return ans;
    }
};