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 {
public:
bool notOperator(string ch) {
return ch != "+" && ch != "-" && ch != "*" && ch != "/";
}

int evalRPN(vector<string>& tokens) {
stack<int> operands;
int ans;
for (int i = 0; i < tokens.size(); i++) {
if (notOperator(tokens[i]))
operands.push(atoi(tokens[i].c_str()));
else {
int operand2 = operands.top();
operands.pop();
int operand1 = operands.top();
operands.pop();
if (tokens[i] == "+")
ans = operand1 + operand2;
else if (tokens[i] == "-")
ans = operand1 - operand2;
else if (tokens[i] == "*")
ans = operand1 * operand2;
else
ans = operand1 / operand2;
operands.push(ans);
}
}
return operands.top();
}
};