leetcode 150. 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

class Solution {
	int my_atoi(string s)
	{
		int re = 0;
		bool positive = true;
		while (s[0] == '-' || s[0] == '+')
		{
			if (s[0] == '-')
				positive = !positive;
			s.erase(0, 1);
		}
		for (int i = 0; i < s.length(); i++)
			re = 10 * re + s[i] - '0';
		return positive ? re : -re;
	}
public:
	int evalRPN(vector<string>& tokens) {
		if (tokens.empty())
			return 0;
		vector<int>nums;
		for (int i = 0; i < tokens.size(); i++)
		{
			if (tokens[i].compare("+") == 0)
			{
				int n1 = nums.back(); nums.pop_back();
				int n2 = nums.back(); nums.pop_back();
				nums.push_back(n1 + n2);
			} 
			else if (tokens[i].compare("-")==0)
			{
				int n1 = nums.back(); nums.pop_back();
				int n2 = nums.back(); nums.pop_back();
				nums.push_back(n2 - n1);
			}
			else if (tokens[i].compare("*") == 0)
			{
				int n1 = nums.back(); nums.pop_back();
				int n2 = nums.back(); nums.pop_back();
				nums.push_back(n1 * n2);
			}
			else if (tokens[i].compare("/") == 0)
			{
				int n1 = nums.back(); nums.pop_back();
				int n2 = nums.back(); nums.pop_back();
				nums.push_back(n2/n1);
			}
			else
				nums.push_back(my_atoi(tokens[i]));
		}
		return nums[0];
	}
};

accepted