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class Solution {
public:
    int calculate(string s) {
        stack<long long int> myStack;
        char sign = '+';
        long long int res = 0, tmp = 0;
        for(int i = 0; i < s.size(); i++) {
            if(isdigit(s[i]))
                tmp = 10*tmp + s[i]-'0';
            if(!isdigit(s[i]) && !isspace(s[i]) || i == s.size()-1) {
                if(sign == '-')
                    myStack.push(-tmp);
                else if(sign == '+')
                    myStack.push(tmp);
                else {
                    int num;
                    if(sign == '*' )
                        num = myStack.top()*tmp;
                    else
                        num = myStack.top()/tmp;
                    myStack.pop();
                    myStack.push(num);
                }
                sign = s[i];
                tmp = 0;
            }
        }
        while(!myStack.empty()) {
            res += myStack.top();
            myStack.pop();
        }
        return res;
    }
};