[LeetCode]150 Evaluate Reverse Polish Notation

public class Solution {
public int evalRPN(String[] tokens) {

// Assumptions...

Stack<Integer> stack = new Stack<>();
for (String s : tokens)
{
if (isop(s))
{
int vb = stack.pop();
int va = stack.pop();
stack.push(calc(va, vb, s));
}
else
{
stack.push(Integer.parseInt(s));
}
}

return stack.pop();
}

private boolean isop(String s)
{
return s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/");
}

private int calc(int a, int b, String op)
{
if (op.equals("+"))
return a + b;
else if (op.equals("-"))
return a - b;
else if (op.equals("*"))
return a * b;
else
return a / b;
}
}