Basic Calculator II.cpp

class Solution {
public:
    int calculate(string s) {
        char op = '+'; 
        int curr = 0; 
        vector<int> stk; 
        
        for (int i = 0; i < s.size(); ++i) {
            
            if(isdigit(s[i])) curr = curr*10 + (s[i] - '0'); 
            
            if(i==s.size()-1 || s[i]=='+' || s[i]=='-' || s[i]=='*' || s[i]=='/'){
                if (op == '+') stk.push_back(curr); 
                else if (op == '-') stk.push_back(curr*(-1)); 
                else if (op == '*'){
                    int num = stk.back();
                    stk.pop_back();
                    stk.push_back(num * curr);
                }
                else if (op == '/'){
                    int num = stk.back();
                    stk.pop_back();
                    stk.push_back(num / curr); 
                }
                
                curr = 0;
                op = s[i];      
            }
        }
        return accumulate(stk.begin(), stk.end(), 0); 
    }
};