227.cpp

class Solution {
public:
    int calculate(string s) {
        s += "+";
        stack<int> st;
        char old = '+';
        for(int i = 0, left = 0; i < s.size(); i++) {
            if (isdigit(s[i]) || isspace(s[i])) continue;
            int val = stoi(s.substr(left, i-left));
            if (old == '+' || old == '-') st.push(old == '+' ? val : -val);
            else st.top() = (old == '*' ? st.top()*val : st.top()/val);
            old = s[i];
            left = i+1;
        }
        int ans = 0;
        while(!st.empty()) {
            ans += st.top();
            st.pop();
        }
        return ans;
    }
};