Evaluatethe value of an arithmetic expression in Reverse Polish Notation. Valid operators are +, -, *, /. Each operand may be an integer or another expression. Some examples:  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

int evalRPN(vector<string> &tokens){    stack<int> stokens;    for (int i = 0; i < tokens.size(); ++ i) {        if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {            int x1 = stokens.top();            stokens.pop();            int x2 = stokens.top();            stokens.pop();            int x3 = caculate(x1, x2, tokens[i]); //计算之后再入栈            stokens.push(x3);        }        else {            stokens.push(atoi(tokens[i].c_str()));        }    }    return stokens.top();}

int caculate(int x1, int x2, string s) {    int result;    if (s == "+")        result = x1 + x2;    else if (s == "-")         result = x1 - x2;    else if (s == "*")        result = x1 * x2;    else {        if (x1 >= x2)            result = x1 / x2;        else            result = x2 / x1;    }    return result;}