LeetCode-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

Solution:
Code:

<span style="font-size:14px;">class Solution {
public:
int evalRPN(vector<string> &tokens) {
const int length = tokens.size();
stack<int> stk;
for (int i = 0; i < length; ++i)
if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {
int second = stk.top();
stk.pop();
int first = stk.top();
stk.pop();
if (tokens[i] == "+")
stk.push(first+second);
else if (tokens[i] == "-")
stk.push(first-second);
else if (tokens[i] == "*")
stk.push(first*second);
else if (tokens[i] == "/")
stk.push(first/second);
} else
stk.push(atoi(tokens[i].c_str()));
return stk.top();
}
};</span>