leetcode 刷题之路 2 Evaluate Reverse Polish Notation

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

逆波兰表达式的求解问题，设置一个栈，遍历逆波兰表达式，遇到数字则入栈，遇到操作符后，栈顶的两个数字出栈，用这两个数字和操作符进行运算所得结果入栈，执行这个操作过程直到遍历完成，最后栈顶的数字就是要求的结果。

my accepted answer:

class Solution {
public:
    int evalRPN(vector<string> &tokens) 
    {
        stack<int> ss;
		for (vector<string>::iterator iter = tokens.begin(); iter != tokens.end(); iter++)
		{
			if (*iter == "+" || *iter == "-" || *iter == "*" || *iter == "/")
			{
				int num1 = ss.top();
				ss.pop();
				int num2 = ss.top();
				ss.pop();

				switch ((*iter)[0])
				{
				case '+':
					ss.push(num1 + num2);
					break;
				case '-':
					ss.push(num2 - num1);
					break;

				case '*':
					ss.push(num2 * num1);
					break;
				case '/':
					ss.push(num2 / num1);
					break;
				}
			}
			else
				ss.push(atoi(iter->c_str()));
		}
		int ret = ss.top();
		ss.pop();
		return ret;
    }
};