LeetCode---2.Evaluate Reverse Polish Notation

class Solution {
public:
	int evalRPN(vector<string> &tokens) {
		while (!nums.empty()) {
			nums.pop();
		}
		vector<string>::iterator iter = tokens.begin();
		while (iter != tokens.end()) {
			string temp = *iter;
			if (isOperator(temp)) {
				int num_2 = nums.top();
				nums.pop();
				int num_1 = nums.top();
				nums.pop();
				int result = calculate(num_1, num_2, temp);
				nums.push(result);
			}
			else {
				nums.push(atoi(temp.c_str()));
			}
			iter++;
		}
		return nums.top();
	}

	bool isOperator(string op) {
		return (op == "+" || op == "-" || op == "*" || op == "/");
	}

	int calculate(int num_1, int num_2, string op) {
		if (op == "+") {
			return num_1 + num_2;
		}
		else if (op == "-") {
			return num_1 - num_2;
		}
		else if (op == "*") {
			return num_1 * num_2;
		}
		else if (op == "/") {
			return num_1 / num_2;
		}
	}
private:
	stack<int> nums;
};