Evaluate Reverse Polish Notation(逆波兰表达式)

Evaluate the 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
说明：逆波兰表达式又叫做后缀表达式。在通常的表达式中，二元运算符总是置于与之相关的两个运算对象之间，所以，这种表示法也称为中缀表示。

class Solution {
public:

    int evalRPN(vector<string> &tokens) {
        int a, b;
        stack<int> s;
        for(int i = 0; i < tokens.size(); i ++)
        {
            if(isdigit(tokens[i][0]) || tokens[i].length() > 1)
            {
                s.push(atoi(tokens[i].c_str()));
                continue;
            }
            a = s.top();s.pop();
            b = s.top();s.pop();
            switch(tokens[i][0])
            {
                case '+': s.push(b + a); break;
                case '-': s.push(b - a); break;
                case '*': s.push(b * a); break;
                case '/': s.push(b / a); break;
            }
        }
        return s.top();
    }
};