利用栈，遇到操作数入栈 ，遇到运算符则从数字栈中取出两个数进行运算并将结果入栈，直至栈中没有操作符，此时栈顶元素即为运算结果

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> num;   //存放操作数的栈
        int n = tokens.size();
        for(int i=0; i<n; i++){
            if(tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "/" || tokens[i] == "*")
            {
                int num2 = num.top(); //这里注意应为num2，在遇到/时作为被除数
                num.pop();
                int num1 = num.top();
                num.pop();
                if(tokens[i] == "+"){
                    num.push(num1 + num2);
                }
                if(tokens[i] == "-"){
                    num.push(num1 - num2);
                }
                if(tokens[i] == "*"){
                    num.push(num1 * num2);
                }
                if(tokens[i] == "/")  num.push(num1 / num2);
            }
            else num.push(stoi(tokens[i]));

        }

        return num.top();

    }
};