class Solution {
public:
    int str_to_int(string s)
    {
        int result = 0;
        if (s[0] == '-')
        {
            for (int i = s.size() - 1, j = 0; i > 0; i--, j++)
            {
                result = result + (s[i] - '0') * pow(10, j);
            }
            result = -result;
        }
        else
        {
            for (int i = s.size()-1 ,  j =0; i >=0; i-- ,j++)
            {
                result = result + (s[i] - '0') *pow(10 ,j);
            }
        }
        return result;
    }
    int evalRPN(vector<string>& tokens) {
        stack<long> stack_t;
        long sum , num_1 ,num_2;
        for( int i=0 ; i<tokens.size();i++)
        {
            if (tokens[i] == "+")
            {
                num_2 = stack_t.top();
                stack_t.pop();
                num_1 = stack_t.top();
                stack_t.pop();
                stack_t.push(num_1 + num_2);
            }
            else if (tokens[i] == "-")
            {
                num_2 = stack_t.top();
                stack_t.pop();
                num_1 = stack_t.top();
                stack_t.pop();
                stack_t.push(num_1 - num_2);
            }
            else if (tokens[i] == "*")
            {
                num_2 = stack_t.top();
                stack_t.pop();
                num_1 = stack_t.top();
                stack_t.pop();
                stack_t.push(num_1 * num_2);
            }
            else if (tokens[i] == "/")
            {
                num_2 = stack_t.top();
                stack_t.pop();
                num_1 = stack_t.top();
                stack_t.pop();
                stack_t.push(num_1 / num_2);
            }
            else
            {
                stack_t.push(str_to_int(tokens[i]));
                // cout << str_to_int(tokens[i]) << endl;
            }
        }
        return stack_t.top();
    }
};

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> st;
        for (int i = 0; i < tokens.size(); i++) {
            if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {
                int num1 = st.top();
                st.pop();
                int num2 = st.top();
                st.pop();
                if (tokens[i] == "+") st.push(num2 + num1);
                if (tokens[i] == "-") st.push(num2 - num1);
                if (tokens[i] == "*") st.push(num2 * num1);
                if (tokens[i] == "/") st.push(num2 / num1);
            } else {
                st.push(stoi(tokens[i]));
            }
        }
        int result = st.top();
        st.pop(); // 把栈里最后一个元素弹出（其实不弹出也没事）
        return result;
    }
};

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<long long> my_stack;
        for(int i=0 ; i<tokens.size() ;i++)
        {  
            if(tokens[i]=="+"||tokens[i]=="-"||tokens[i]=="*"||tokens[i]=="/") 
            {
                long long tmp1 = my_stack.top();
                my_stack.pop();
                long long tmp2 = my_stack.top();
                my_stack.pop();
                // cout<<tmp2<<' '<<tmp1<<endl;
                if(tokens[i] == "+")
                    my_stack.push( tmp2+tmp1);
                else if(tokens[i] == "-")
                    my_stack.push( tmp2-tmp1);
                else if(tokens[i] == "*")
                    my_stack.push(tmp2*tmp1);
                else if(tokens[i] == "/")
                    my_stack.push(tmp2/tmp1 );
            }else
                my_stack.push(stoi(tokens[i]));
        }
        return my_stack.top();
    }
};