逆波兰求表达式

class Solution {
public:
    int evalRPN(vector<string>& tokens) 
    {
        int ret = 0;
        stack<int> st;
        int x, y;
        for (int i = 0; i < tokens.size(); i++)
        {
            if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/")
            {
                x = st.top();
                st.pop();
                y = st.top();
                st.pop();
                if (tokens[i] == "+") st.push(y + x);
                else if (tokens[i] == "-") st.push(y - x);
                else if (tokens[i] == "*") st.push(y * x);
                else st.push(y / x);
            }
            else
                st.push(stoi(tokens[i]));//stoi将string字符串变成十进制数字
        }
        return st.top();
    }
};